Defeating the current world boxing champion was undoubtedly the of his career.1. zephyr2. zenith3. zohar4. zealot

The zenith of his career occurs only by defeating the current World Boxing Champion.

Explanation:

The synonyms of the word zenith is crowning point or high point.

In this sentence, the career of a boxer becomes optimum only when he/she defeats or outcomes the current World Boxing Champion.  

The other words are not suitable for this sentence because the synonyms of the words in other option does not complete the meaning of the sentence.

The synonym of the words are given below:The synonyms of zephyr is light air stream or breeze. The word Zohar is from the language Hebrew which is used in Israel. This word considerable refers to a literal work which gives view about the nature of god and soul; and origin of the universe. The synonyms of zealot is being uncompromising in a religion or an idea. It also refers the religious movement conducted by Jews against Roman Empire.

A,) cause a & b are equal & both gives 180°

It is c. naturally in the environment