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However, this solution is not terribly efficient. For each character in the string, we're iterating over the string again with our calls to IndexOf, so our time complexity is \$O(n^2)\$. @BlueTrin's answer shows how you can trade space to reduce the time complexity to \$O(n)\$, so I won't go over that here.

However, this solution is not terribly efficient. For each character in the string, we're iterating over the string again with our calls to IndexOf, so our time complexity is \$O(n^2)\$.

In fact, we've basically just written this:

var maxCount = 0;
foreach (var c in input)
{
    var count = 0;
    foreach (var d in input)
    {
        if (c == d)
        {
            count++;
        }
    }
    
    maxCount = Math.Max(count, maxCount);
}

return maxCount;

@BlueTrin's answer shows how you can trade space to reduce the time complexity to \$O(n)\$, so I won't go over that here.

var maxCount = 0;
foreach (var c in input)
{
    // TODO: Count occurrences of c in input.
    maxCount = Math.Max(count, maxCount);
}

var maxCount = 0;
foreach (var c in input)
{
    var count = 0;
    var i = input.IndexOf(c);
    while (i != -1)
    {
        // TODO: Keep counting occurrences of c.
    }
    
    maxCount = Math.Max(count, maxCount);
}

var maxCount = 0;
foreach (var c in input)
{
    // TODO: Count occurrences of c in input.
    maxCount = Math.Max(count, maxCount);
} 

return maxCount;

var maxCount = 0;
foreach (var c in input)
{
    var count = 0;
    var i = input.IndexOf(c);
    while (i != -1)
    {
        // TODO: Keep counting occurrences of c.
    }
    
    maxCount = Math.Max(count, maxCount);
}

return maxCount;

The basic structure of your code is good. That is, the part that effectively goes

There is a pretty big bug in your code, as the following will not terminate:

Console.WriteLine(GetMaximumOccurence(" "));

Can you see why?

The basic structure of your code is good. That is, the part that effectively goes