This sentence gives a number of passengers killed in a bus accident, and we must check if the words used with that number are grammatically correct.
The error lies in part (a), "Not less than", because "less" should be "fewer" when talking about a countable noun like passengers.
Let's summarize:
So the correct option is (a), "Not less than".
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?