Actually, in normal, casual English conversation, either A or D would be the most appropriate response, and are the phrases you'd hear most often in that type of situation. "I'd be very happy to give you a hand" sounds extremely formal and stilted and is found more in text books than in actual conversation.
In AmE, the most common repsonses when someone says "if you don't mind" are:
Not at all.
Of course I don't mind.
Don't mind a bit.
No, I don't mind.