What natural plant hormone is commonly used to ripen fruits?

The correct answer is ethylene, which is a natural plant hormone that plays a crucial role in the ripening process of fruits. Ethylene is a gaseous hormone produced by many fruits as they mature, and it initiates a series of biochemical processes that lead to ripening. This includes changes in color, softening of the flesh, and the development of flavor and aroma compounds, making the fruit more appealing for consumption.

When ripe fruit releases ethylene, it can also stimulate nearby fruits to ripen, which is why ethylene is often utilized in storage and shipping practices to control and synchronize the ripening process. This attribute makes it particularly valuable in agricultural practices and the fruit industry, ensuring that fruits reach consumers at their peak quality.

The other options presented do not serve the same function as a ripening agent in fruits. Methanol and ethanol are types of alcohol, which do not facilitate the ripening process. Acetyl acid (commonly found in vinegar) is not a known hormone in fruit ripening. Thus, the importance of ethylene as the main natural hormone for ripening highlights its key role in agriculture and food science.