The correct option is C Would
Here, 'would' is correct because it is used in polite offers or invitations.
'Had' is incorrect because it is used instead of 'if' to begin a clause which refers to a situation that might have happened but did not.
How' as an adverb is used to imply in what way or manner.
'Why' is incorrect because it is used in questions to ask the reason for or purpose of something.