The idiom "Buy a lemon" refers to purchasing something, particularly a vehicle, that turns out to be defective or constantly problematic. It is a colloquial expression commonly used in the automotive industry to describe cars that appear to be fine at the time of sale but soon reveal issues.
This idiom comes from American slang where a "lemon" is used as a nickname for a faulty product, most often a car.
The answer key marks option A here, though the widely accepted dictionary meaning of "buy a lemon" actually matches option D (a faulty, problem prone purchase); keep the keyed answer as given, but this is worth double checking against a dictionary if you see it in another paper.