Answer:
The pair of variables that show a clear causal link are the number of lights switched on in a room and the room's brightness
Explanation:
Clear casual link indicates two variables that have a completly and unequivocal relation, meaning that changes in one variable produce automatically and with no doubt a change in the other.
Case A does not have this link as these two variables have no relation at all. A person from certian state may have any type of academic degree.
Case B Cost of aviation fuel may be explained by other variables like international prices and flight duration may be explained also due to climate.
Case D- a person may have a pet even under low income levels.
Case C is the only adequate relation, as turning lights on will always increase room's brightness.