Find the synonym of the underlined word:
Tides are caused by the gravitational influence of the moon on ocean level.

We've to find the meaning of influence and the meaning of words that are given in the options. And both the words should have the same meaning or the exact sense. 
Option A) elaboration – is an incorrect answer because the meaning of elaboration is ‘ the gradational development of commodity ’ and therefore this isn't the reverse of influence. therefore, this is an incorrect answer because the meaning of the words ‘ elaboration ’ and ‘ influence ’ doesn't match. 
Option B) compulsion – is the incorrect answer because the meaning of compulsion is ‘ the practice of forcing someone to act as an involuntary manner by use of trouble or force ’. So, compulsion isn't a reverse with influence. therefore, this is an incorrect answer because the meaning of the words ‘ compulsion ’ and ‘ influence ’ doesn't match. 
Option C) Un involvement – is an incorrect option because un involvement means ‘ they aren't involved in anything ’. Then in the judgment moon is involved in causing runs. therefore, this is an incorrect answer because the meaning of the words ‘ un involvement ’ and ‘ influence ’ doesn't match. 
Option D) Impact – is the correct answer the meaning of impact is analogous to the meaning of influence. Both words have analogous meanings and therefore, this is the correct answer. 
runs are caused by the gravitational impact of the moon on the ocean position.