Final answer:
The word most similar in meaning to 'vital' is 'necessary', which means something that is absolutely essential or critical for life or functionality.
Step-by-step explanation:
In response to the question, which of the following is most similar in meaning to the word vital, the correct answer is a. necessary. The word vital means something that is absolutely essential or important. It is essential to the survival or functioning of something. An example of using vital in a sentence could be, 'Drinking water is vital for all living organisms,' illustrating that water is necessary for life.