这一题应该不能选择C for，表示在某一方面应该选择B in.
essential
n. 本质；要素；要点；必需品
adj. 基本的；必要的；本质的；精华的
例句：
They believe it is essential to defy convention.
他们认为对抗流俗是绝对必要的。
essential的相关短语有：
essential oil 香精油，精油
essential condition 必要条件
essential hypertension 原发性高血压；特发性高血压
essential element 必需元素
essential component 主要部件；主要成分
个人觉得选for也没有问题，下面为朗文的一些供参考资料：
es‧sen‧tial
1 extremely important and necessary
--essential for/to
A good diet is essential for everyone.
--it is essential (that)
It is essential that our pilots are given the best possible training.
--it is essential to do something
It is essential to book in advance.
Window locks are fairly cheap and absolutely essential.
Even in small companies, computers are an essential tool.
2 the essential part, quality, or feature of something is the most basic one:
--The essential difference between Sam and me was the fact that I took life seriously.
--The essential point of relay racing is that it is a team effort.
下面为牛津的解释，觉得for也没有问题
参考资料：http://www.oxforddictionaries.com/definition/learner/essential
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