using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<TABLEA> tableas = new List<TABLEA>()

        { 

            new TABLEA(001, "iphone", 9890),

            new TABLEA(002, "iphone pro max", 9890),

            new TABLEA(003, "iphone se", 9890),

            new TABLEA(004, "iphone", 7847),

            new TABLEA(008, "samsung", 9432),

            new TABLEA(009, "samsung s10", 9432),

            new TABLEA(010, "orange", 34)

        };

        var result = tableas

            .Where(x => x.Id == 1)

            .GroupBy(x => x.regNumber)

            .Select(x => new {Description = x.First().Desc, regNumber=x.Key, TotalCount = x.Count()})

            .OrderByDescending(x => x.TotalCount)

            .First();

        Console.WriteLine($"{result.Description} {result.regNumber} {result.TotalCount}");

    }

}

​

public class TABLEA

{

    public int Id { get; set;}

    public string Desc { get; set;}

    public int regNumber { get; set;}

    public TABLEA(int id, string desc, int regNumber)

    {

        this.Id = id;

        this.Desc = desc;

        this.regNumber = regNumber;

    }

}

​