Explanation:

- Here’s the thing: Only one of these is actually used for fruit ripening.

- Methylene: You might see this in labs, but it has nothing to do with ripening fruit.

- Ethylene: This is the gas plants naturally produce. It signals fruit to ripen—bananas, mangoes, you name it. Sometimes it’s even used commercially to ripen fruit faster.

- Acetone: That’s nail polish remover, not a fruit ripener.

- Methane: You’ll find this in natural gas, not in fruit bowls.

So, the right answer is Option 2: Ethylene.