Рecursion

        static StringBuilder WordSearch(int letterNumber, StringBuilder word, char[] alphabet)
        {
            StringBuilder result = new StringBuilder();
            if(letterNumber < 1)
            {
                return word;
            }
            foreach(var a in alphabet)
            {
                word[letterNumber-1] = a;
                result.Append(WordSearch(letterNumber-1, word, alphabet).ToString() + Environment.NewLine);
            }
            return result;
        }