Correct Answer: (b) least expensive
Solution:'Superlative degree adjective' denotes the form of an adjective that expresses the highest or a very high degree of quality of a thing, person, etc. For example, best, worst, greatest, most, least, etc. However, we always place the definite article 'the' before any superlative adjective. Similarly, in the given sentence, 'least' is the superlative degree adjective. Hence, 'the least expensive' is the most appropriate answer.