One of the easiest and quickest ways to get ______ is by consciously building up your knowledge of words.

题目考察的是词义填空。根据句子结构，“One of the easiest and quickest ways to get _ is by consciously building up your knowledge of words.” 可以看出，这里需要填入一个词，与“词汇量”或“语言能力”相关。结合上下文，句子的核心在于“通过有意识地积累词汇来获得某种能力或知识”。

分析选项，最合适的词是“vocabulary”（词汇量）。完整的句子应为：
“One of the easiest and quickest ways to get vocabulary is by consciously building up your knowledge of words.”

这里，“vocabulary”表示“词汇量”，与“knowledge of words”相呼应，符合语境。其他可能的词如“knowledge”或“understanding”虽然也有可能，但不如“vocabulary”更贴合“building up your knowledge of words”的语境。

最终答案是：vocabulary。