Select the most appropriate option to fill in the blank.

She refused to back ______, so she was fired from the company.

down

The most appropriate option to fill in the blank is down.

The sentence is about how she refused to back down, so she was fired from the company. The word "back down" means "to give up or withdraw from a position or demand." This is the most appropriate word because it accurately describes how she refused to change her mind or give in to the company's demands.

The other options are not as appropriate.

• Over means "across or on the other side of." This does not make sense in the context of the sentence, because she was not refusing to go over something.
• Upon means "on or against." This does not make sense in the context of the sentence, because she was not refusing to go upon something.
• Into means "in or to the inside of." This does not make sense in the context of the sentence, because she was not refusing to go into something.