Which sentence is a conditional sentence?

A conditional sentence is a two-part statement that expresses a condition and a consequence, typically using "if" to introduce the condition. In the selected sentence, "If it rains, we will stay indoors," it clearly presents a condition (the possibility of rain) that leads to a specific outcome (staying indoors). This structure is characteristic of conditional sentences, as it sets up a scenario where one action depends on another.

The other sentences do not follow this conditional structure; instead, they make straightforward statements or suggestions without indicating a cause-and-effect relationship. Hence, they lack the conditional elements that define the correct answer.