Answer:
84%
Explanation:
"What percent of 25 is 21"
"of" can be translated to a multiplication sign
"is" can be translated to an equal sign
And, we can call "what percent" as P, since we want to solve for this.
So, we have:

Solving for P, and then multiplying by 100 to get the percentage, we have:

84% is the correct answer.