Which one of the following choices best represents a potential stressor?
a. a decrease in gas prices
b. a long-awaited salary raise
c. a surprise gift
d. an unexpected visit

I would say that a) decrease in gas is positive (unless you sell the gas: then it is a potential stressor)

b) this would be a positive surprise, not a stressor

c) this would also be a positive surprise, not a stressor

d) I think that this would be a potential stressor: especially if you don't feel comfortable with the people who visit, or would have preferred to clean up the flat, or don't have time for the visit: this would be my answer.