Capital letter for "I" - writing "i'm" is a spelling mistake. Your answer/reply is late, so... "I'm sorry for my late answer." "I apologise for the late reply." (formal/business) "I'm sorry for answering late." "Please excuse my tardy reply." (I wanted a sentence which uses "pardon" or something similar, but this sentence would make you sound like an old aristocrat))

I'm sorry about my late answer, or, pardon my late answer.