Answer:
B. The word "less" is an adjective that tells how much.
Step-by-step explanation:
Adjectives act as a modifier that aims to outline the quality or state of a noun or pronoun. As per the statement has given the adjective "less" that states the comparison of time which can not possibly relate to "kind", "extent" or "way" here as it exemplifies that 'how much' less time has he taken for studying as compared to the time he thought up. Thus, the second option is appropriate as it modifies the noun "time" purposely to show the effective comparison.