After she received a free hand-painted Christmas ornament from a religious organization, Mrs. Montevecchi felt obligated to mail a cash donation to the organization. Her response to the free gift best illustrates the impact of what?

The answer is: The reciprocity norm

According to The reciprocity norm, people are more inclined to treat other people with kindness if those people have done something that benefit us in the past.

This is what happen to Mrs. Montevecchi. After receiving a free hand-painted ornament, she will become more likely to give the donation because she wanted to repay the 'benefit' that is given by the organization.