I believe the answer is (C), because spellcheck corrects your grammar, teaching you how you can improve on your grammar next time.
(B) isn't possible, because that's what spellcheck does! Corrects grammar and spelling errors.
(D) isn't right, because that's not what spellcheck does. It corrects and grammar and spelling errors.
(A) I don't believe it's right, because sometimes, you must find your own mistakes. Spellcheck doesn't fix everything.