Question

which best completes the sentence? longer, most long, longest mammoth cave in kentucky is the worlds ______ cave system.

Explanation:

The sentence compares Mammoth Cave to all other cave systems globally, requiring the superlative form of "long". "Longer" is comparative (for two items), "most long" is incorrect grammar for this adjective, and "longest" is the correct superlative form for the adjective "long".

Answer:

longest