Answer:
A. Riley didn't understand the math problem so she vexed over the
Step-by-step explanation:
Option A uses the word vexed correctly.
It says Riley did not understand the math problem so she vexed over the answer for at least one hour.
To be vexed means to be angered, frustrated, to annoyed or even worried over something.
This answer option tells us that Riley's inability to understand the mathematics problem caused her to be annoyed and frustrated over what should be the solution for at least one hour.