This isn't really a good application of the contrapositive.
Instead, look at what's being described.
We have a situation where the housing market is bad. BUT, in contrast to that, online real estate brokers are thriving. We need to resolve this paradox.
All answer choices explain this except for one.
D says that in bad times, online brokers advertise -- which, in turn, brings in customers, and explains their success in a bad market.
C says that online brokers are cheaper. This also could explain their success.
B, on the other had, talks about a situation when the economy is strong. This is not what the argument's about, and so doesn't explain the online brokers' success in a down market.