Which scale degree should a listener use to determine the mode of a selection?
a. fifth
b. fourth
c. first
d. third?

A listener should listen to the third degree to figure out the mode of a selection. This is where the naturally occurring half steps are. The third degree is the halfway point, and this can show one a starting point to determine if a mode is a higher or lower fluctuation.

That is true. hope it ! : )