The correct answer is A.

The conclusion of the argument is that "people must not be as interested in buying new homes as they were even six months ago." Why? Because of several factors (developers not buying land, contractors without work, banks issuing fewer mortgages) that the author assumes result from the supposed lack of interest in buying new homes. Choice A suggests, though, that there may be an alternative explanation for all of those factors: interest rates are too high for most people. So even though they remain interested in buying homes, they simply cannot afford to do so and the whole housing market has slumped accordingly.