For problem A, you are looking for odd rows and column, so the indexes should go for 1 with an increment of 2.
Like
A = X(1:2:end,1:2:end); % end simply represents the end
For B: you are excluding the first and last, so the indexes should exclude those.
B = X(2:end-1,2:end-1);
For C, I'd say first write down the indexes you're looking for on a piece of paper and see if you see a pattern. Post, even if it is wrong, and I'll tell you how to do it.
Sometimes it is easy to write out on a a piece of paper the indexes because it help you visualize.
