Answer:
The answer is C. "churned the water white" lets us see how hard and fast the men are rowing.
Step-by-step explanation:
When you observe a boat or ferry moving fast on water, it leaves a white trail behind. Or a consistent fast disturbance on a body of water, the water tends to look white. This is due to the fast movement on water.
In the excerpt above, "churning the water white" alludes to a similar situation. They used their oars to row, with consistent, hard, fast strokes as described with "stroke on stroke".
Thus, in the sentence above, "churned the water white" lets us see how hard and fast the men are rowing.