Hurry! nwhich best explains why international trade agreements are beneficial for developing economies?

The correct answer is the second: "They can help countries compete with the United States".

When a trade agreement is reached between two or several countries, this means that trade barriers are removed between the parties. Therefore, goods and services can go in and out without having to pay taxes at the border (this type of taxes which burden imports/exports are called duties). Duties are only one type of trade barrier, there are many others.

When the US reaches a trade agreement with a foreign country, goods from there can freely enter the borders and compete in equal conditions with goods produced by US industries.

This can be harmful for the US economies, when foreign imports are produced in much cheaper conditions than the domestic ones. This is common in some sector, such as the textile industry. Those cheaper imports are comercialized at a lower price and consumers would prefer them rather than more expensive ones produced in the US.