Answer is (B), but how about (A) or (C) as an answer?
M: I am ordering an exercise bike that I saw on the Home Shopping Network.
W: Well, our treadmill in the living room hasn't been used for years.
I think buying more sports equipment will be wasting money
M: I promise I will work out with it everyday.
W: __________________________________________________ _________
(A) Better late than never.
(B) Are you sure you can make it worthwhile?
(C) You said it. You should work out regularly.
(D) Mind your own business.
C doesn't have anything to do with the conversation about new exercise equipment.
The man promises to work out with "it" meaning the new bike. A doesn't work because "better late then never" refers to something he could have already been going - and working out with a new bike that he doesn't yet have isn't something he could have been doing already.