Which revision fixes the dangling modifier in the sentence?
rowing through the rapids, the canoe began to fill with water.

the canoe began to fill with water when rowing through the rapids.

the canoe began to fill with water, rowing through the rapids.

when rowing through the rapids, the canoe began to fill with water.

when we were rowing through the rapids, the canoe began to fill with water.

The revision that fixes the dangling modifier in the sentence is the last option:
When we were rowing through the rapids, the canoe began to fill with water.

A dangling modifier is called that way because we don't exactly know what it refers to. The way it is written, the first sentence would mean that the canoe was rowing, and not the people inside of it.

The central idea, which the author develops throughout 'The Dark Game' is that during the Civil War, ordinary individuals who wanted to aid the war effort acted as spies.