C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;

					
public class Program
{
	public static void Main()
	{
		List<Student> students = new List<Student>
		{
			new Student { ID = 1, Name = "Alice Johnson", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" }, new Course { CourseID = "C102", CourseName = "Science" } } },
			new Student { ID = 2, Name = "Bob Smith", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" }, new Course { CourseID = "C103", CourseName = "History" } } },
			new Student { ID = 3, Name = "Charlie Brown", Courses = new List<Course> { new Course { CourseID = "C102", CourseName = "Science" } } },
			new Student { ID = 4, Name = "Diane Miller", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" } } }
		};
		
		// Flatten the list to get all student-course enrollments
		var courseEnrollments = students.SelectMany(s => s.Courses, (s, c) => new { s.ID, s.Name, c.CourseID, c.CourseName }).ToList();

		// Step 1: Identify courses with the highest student enrollment
		var coursesWithHighestEnrollment = courseEnrollments
			.GroupBy(e => e.CourseID)
			.Select(group => new { CourseID = group.Key, EnrollmentCount = group.Count() })
			.OrderByDescending(result => result.EnrollmentCount)
			.ToList();

		int maxEnrollment = coursesWithHighestEnrollment.First().EnrollmentCount;
		var highestEnrollmentCourses = coursesWithHighestEnrollment.Where(c => c.EnrollmentCount == maxEnrollment);

		Console.WriteLine("Courses with the Highest Student Enrollment:");
		foreach (var course in highestEnrollmentCourses)
		{
			Console.WriteLine($"{course.CourseID} with {course.EnrollmentCount} enrollments");
		}

		// Step 2: List all students enrolled in those courses
		var studentsInHighEnrollmentCourses = courseEnrollments
			.Where(e => highestEnrollmentCourses.Select(c => c.CourseID).Contains(e.CourseID))
			.Distinct()
			.ToList();

		Console.WriteLine("Students in High Enrollment Courses:");
		foreach (var student in studentsInHighEnrollmentCourses)
		{
			Console.WriteLine($"- {student.Name} in {student.CourseName}");
		}

		// Step 3: Calculate the total number of unique students across all the highest enrollment courses
		var uniqueStudentCount = studentsInHighEnrollmentCourses.Select(s => s.ID).Distinct().Count();

		Console.WriteLine($"Total Number of Unique Students in High Enrollment Courses: {uniqueStudentCount}");


	}
}

public class Student
{
    public int ID { get; set; }
    public string Name { get; set; }
    public List<Course> Courses { get; set; }
}

public class Course
{
    public string CourseID { get; set; }
    public string CourseName { get; set; }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
​
                    
public class Program
{
    public static void Main()
    {
        List<Student> students = new List<Student>
        {
            new Student { ID = 1, Name = "Alice Johnson", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" }, new Course { CourseID = "C102", CourseName = "Science" } } },
            new Student { ID = 2, Name = "Bob Smith", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" }, new Course { CourseID = "C103", CourseName = "History" } } },
            new Student { ID = 3, Name = "Charlie Brown", Courses = new List<Course> { new Course { CourseID = "C102", CourseName = "Science" } } },
            new Student { ID = 4, Name = "Diane Miller", Courses = new List<Course> { new Course { CourseID = "C101", CourseName = "Mathematics" } } }
        };
        
        // Flatten the list to get all student-course enrollments
        var courseEnrollments = students.SelectMany(s => s.Courses, (s, c) => new { s.ID, s.Name, c.CourseID, c.CourseName }).ToList();
​
        // Step 1: Identify courses with the highest student enrollment
        var coursesWithHighestEnrollment = courseEnrollments
            .GroupBy(e => e.CourseID)
            .Select(group => new { CourseID = group.Key, EnrollmentCount = group.Count() })