Your click-through rate measures how many people click a link in your post

Divide the total number of clicks on a link in a post by the total number of impressions
Divide the number of conversions you received against the number of clicks you received on your links
Divide the number of conversions you received against the total engagement rate
Divide the total number of clicks on a link in a post by the total reach

The correct answer is:

Divide the total number of clicks on a link in a post by the total number of impressions.