Oh thank you so much !
Placed the c = c +1; behind the array and it works fine now !

I've modified it

static int Fonc_question(int n) // Pose un nombre n de question
        {
            int[] questions = new int[n];
            int q = n; // to make the moy (average)
            int r = n; // to make the sum
            int c = 0; // Will get (c = n) numbers 
            int som = 0; // sum
            int moy = 0; // average
                int rec =0;
            while (n > 0)
            {
                c = c + 1;
                rec +=1;
                    Console.WriteLine("Nombre " + (c) + ":");
                    questions[c] = Convert.ToInt32(Console.ReadLine());
                n=n-1;
            }
            if (n <= 0)
            {
                while (r > 0)
                {
                    r = r -1;
                    som += questions[r];
                }
                moy = som / q;
                Console.WriteLine(moy);
            }
            Console.ReadLine();
            return n;
        }
        static void Main(string[] args)
        {
            int nombre;
            Console.WriteLine("Combien de nombres ?");
            nombre = Convert.ToInt32(Console.ReadLine());
            Fonc_question(nombre);
        }
    }

The code works fine until this line
questions[c] = Convert.ToInt32(Console.ReadLine());

But sadly it seems to work a bit aswell so I don't get it.

Example:

I enter 3 (so i need to enter 3 times a number)
It will prompt me 3 times and once the 3rd number is entered it shows me an error underlining this statement.
Saying: "Index was outside the bounds of the array."

Oh thanks ! Stupid error...

Thanks all done. Now I've ran into another problem but I'll make a new post since it's not really related.

Problem solved.

                lettre = (char)rot13;
                Console.Write(lettre);

Works perfectly. However it display each new string one under another, how can I manage not to do this ? I wrote "Console.Write()" and not "Console.WriteLine()" but doesn't seem to change anything...