The correct answer is option 1 - Acetylene. Acetylene is a nontoxic gas that helps in the formation of enzymes which ripen fruit.

Option 2 - Ethane is not the correct answer. Ethane is not involved in the ripening of fruit.

Option 3 - Methane is not the correct answer. Methane is also not involved in the ripening of fruit.

Option 4 - Carbon dioxide is not the correct answer. Although carbon dioxide is involved in fruit ripening to some extent, it is not a nontoxic gas. It can be harmful in high concentrations.

It is important to note that the ripening of fruit is a natural process that involves the release of certain gases, including acetylene, ethylene, and carbon dioxide. These gases help in the breakdown of complex molecules in the fruit, resulting in the softening, color change, and flavor development associated with ripening. Acetylene, in particular, plays a role in initiating the ripening process by triggering the production of enzymes that break down carbohydrates and other compounds in the fruit.