What can be a simple effect of a descending current?

Correct answer: Tendency of sinking

A descending current is a movement in which material travels downward. In a broad Earth-science explanation, descending material may sink because it becomes cooler and denser, and at a plate boundary it can be associated with subduction or downward movement. The simplest effect named in the options is therefore a tendency toward sinking.

Option A directly matches this meaning. Rain stopping is a weather statement and does not necessarily result from a descending mantle current. A river becoming sweet and a language changing have no physical connection with convection in Earth’s interior. Since the question asks for a simple effect, no detailed claim about every descending current is needed; downward movement and sinking are the relevant ideas.