Based on its structure, which word most likely means "someone who takes advantage of circumstances”?

Opportunist would be a good word to replace the sentence and you can describe that person as bold, daring, pushing, optimistic, corrupt, selfish, narcissistic, sociopath, etc. just depends on the person's personality and the situation, and their motivation for their actions. Also put this in language next time. 

The theory is land mass basically what they said.