Question

several taxi drivers drove up to the barricades and honked their horns in a vain effort to persuade demonstrators to get off the road. in the sentence above, vain has the same or almost the same meaning as: viscous energetic fruitless dangerous

Explanation:

The word "vain" in the context means without result or ineffective. "Futile" also means having no effect or result.

Answer:

C. futile