Program.cs

using System;
using System.Linq;

class MainClass {
    static readonly char[] ee = {'b', 'c', 'd', 'e', 'g', 'p', 't', 'v', 'z'};
    static readonly char[] ay = {'a', 'h', 'j', 'k'};
    static readonly char[] ess = {'f', 's', 'x'};
    static readonly char[] ii = {'i', 'y'};
    static readonly char[] em = {'m', 'n'};
    static readonly char[] u = {'q', 'u'};
    
    public static void Main (string[] args) {
        Console.Write("Enter text: ");
        var UserInput = Console.ReadLine();

        Console.WriteLine();
        Console.WriteLine("We're going to test every possibility. This may take a while...");
        Console.WriteLine();

        foreach (char c in UserInput) {
            Console.WriteLine("==========");
            Console.WriteLine(c);

            var chars = GetAllPossibleChars(char.ToLower(c));

            if (chars == null) {
                continue;
            }

            if (char.IsUpper(c)) {
                chars = CapitaliseCharArray(ref chars);
            }
            
            Console.WriteLine(chars);
        }
    }

    static char[] GetAllPossibleChars(char c) {
        char[] array;

        c = char.ToLower(c);
        Console.WriteLine(c);

        if (ee.Contains(c))
            array = ee;
        else if (ay.Contains(c))
            array = ay;
        else if (ess.Contains(c))
            array = ess;
        else if (ii.Contains(c))
            array = ii;
        else if (em.Contains(c))
            array = em;
        else if (u.Contains(c))
            array = u;
        else {
            Console.WriteLine("YEET");
            return null;
        }

        return array;
    }

    static char[] CapitaliseCharArray(ref char[] array) {
        var i=0;

        foreach (char c in array) {
            array[i] = char.ToUpper(c);
            i++;
        }

        return array;
    }
}