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 the most typical stressor here is an unexpected visit. Starting from you feeling uncomfortable about not preparing (dirty flat, etc), you might not want to meet those people and might feel stressed about how to tel them this.

The other options are potential "positive" surprises.

C. An unexpected visit