Find a palindromic string in C
The amount of comments you use is more distracting than helpful. Good comments are those that explain what the overall logic of a block of code is, they don’t explain each line of code. Code is supposed to be self-documenting, that is, the code should explain itself. You accomplish this by using good variable names and good function names, which explain their own meaning.

Also, you need to use the functionality in the standard library. For example, to test whether a character is a normal letter or not, use isalpha rather than the four comparisons you’re using (against 'a', 'z', etc.)