Find char which appears maximal number of times
According to a modern C# and LINQ you could try to "shock" interviewer with solving in a row:

var res = File.ReadAllText(path)
            .Where(ch =&gt; char.IsLetter(ch))
            .GroupBy(x =&gt; x)
            .OrderByDescending(x =&gt; x.Count())
            .First()
            .Key;