Choose the grammatically correct sentence:

The correct answer is Option (4) → The new scooter uses less petrol than the old one.

Less with uncountable nouns, fewer with countable nouns:

• Less is used with things we cannot count, like water, petrol or sugar. Fewer is used with things we can count, like people or chairs.
• “Petrol” cannot be counted one by one. So “less petrol” is correct.

Why the Other Options are Incorrect?

Option 1: There were less people at the Dussehra mela this year because of the rain. — Incorrect. “People” can be counted. So it takes “fewer”, not “less”.
Corrected: There were fewer people at the Dussehra mela this year because of the rain.

Option 2: Fewer water reached the farms at the end of the canal this summer. — Incorrect. “Water” cannot be counted. So it takes “less”, not “fewer”.
Corrected: Less water reached the farms at the end of the canal this summer.

Option 3: There was not many traffic on the service road this morning. — Incorrect. “Traffic” cannot be counted. So it takes “much”, not “many”.
Corrected: There was not much traffic on the service road this morning.