Unless we ___ throwing garbage into the river, it will be polluted.

The word "unless" introduces a condition. The sentence suggests that pollution can be prevented only if we change our behavior. The correct verb here is "stop"—present tense, as is usual after "unless." So the sentence means: if we don’t stop throwing garbage into the river, it will become polluted. “Stop” implies halting a harmful action, making it the most logical and grammatically correct choice here.