This sentence describes how quickly the mosquitoes started biting after sunset, and we need to check the grammar of the negative expression used at the start.
The error lies in part (a), "The sun hadn't hardly", because "hardly" is already a negative word, and pairing it with "hadn't" creates an incorrect double negative.
Let's summarize:
So the correct option is (a), "The sun hadn't hardly".
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?