One way to understand thousands of new words is to gain __C___good knowledge of basic word formation.
A.\ B.the C.a D.one
翻译：理解成千上万新的词汇需要很好的掌握基本的单词成分.
(one) way 是主语,to understand thousands of new words 是定语,修饰one way ,is 谓语,to gain a good knowledge of basic word formation接下来的是表语,所以选C