A brownout can be defined as a:

A.
Prolonged low voltage.

B.
Prolonged power loss.

C.
Momentary high voltage.

D.
Momentary low voltage.

Explanation:
The correct answer is “Prolonged low voltage”.
Answer “prolonged power loss” is a blackout.
Answer “momentary low voltage” is a sag.
Answer “momentary high voltage” is a spike.