The correct option is D In crushing
Let’s examine the sentence: “When it was feared that the serfs might go too far and gain their freedom from serfdom, the Protestant leaders joined the princes at crushing them.” The gerund “crushing” here means “defeating” and the preposition used with “defeating” is “in.” Hence, the correct sentence would be, “When it was feared that the serfs might go too far and gain their freedom from serfdom, the Protestant leaders joined the princes in crushing them.” So, Option B is the correct answer.