Which activity increases when fertile plains form along rivers?

Correct answer: Agriculture

Rivers often deposit fine soil and minerals along their banks and across nearby lowlands. Over time, these deposits can create fertile plains with soil that supports the growth of crops. When land is fertile and water is available, people are more likely to cultivate it. Therefore, agriculture increases in such areas, making option A correct. Space travel, volcanic eruptions and desert expansion do not result directly from the formation of fertile river plains.

The reasoning is based on the relationship between rivers, soil and farming. Flowing water carries sediment, and slower water can leave this material on the surrounding land. The resulting plain may be level, moist and rich in nutrients, all of which are useful for cultivation. This does not mean every riverbank is always safe or suitable for farming, because floods may occur. However, among the given choices, agriculture is the activity most directly encouraged by fertile plains, so option A is the correct choice.