Step 1: Identify the idiom "cost an arm and a leg," which is a well known way of describing something that costs a very large amount of money, using the exaggerated image of paying with body parts.
Step 2: Match this meaning to the options. "Negligible" means insignificant or almost nothing, the opposite of what is meant. "Reasonable," "Cheap," and "Economical" all describe a price that is fair or low, which again is the opposite of the idiom's meaning. "Exorbitant" means unreasonably or excessively expensive, which is exactly what "cost an arm and a leg" conveys.
Step 3: Since the sentence is about a car that cost a great deal of money, the correct word is the one meaning excessively costly.
Answer: (b) Exorbitant