Select the most appropriate synonym of the given word:

APEX

The correct answer is Pinnacle.

Explanation:
To select the most appropriate synonym, let's break down the meaning of the given word alongside each of the options:

• APEX: Refers to the highest point, summit, or peak of something (for example, the apex of a mountain or the apex of one's career).

• Pinnacle: Means the most successful point, culmination, or the highest physical peak/top of a structure or mountain. Since both "apex" and "pinnacle" denote the ultimate highest point, Pinnacle is the most accurate synonym of APEX.

Analysis of the other options:
• Nadir: The lowest point of anything (this is the antonym of apex).
• Genesis: The origin, creation, or beginning of something.
• Cornerstone: An important quality, foundation, or essential base upon which something is built.