Answer:
A.
Explanation:
A gerund is basically a verb that serves as a noun.
Every complete sentence consists of a main idea and a verb. If you split the sentence into its two parts, you get
"Using energy-efficient light bulbs /// has been helping people to cut down on waste."
The verb part of the sentence is has been helping people to cut down on waste. But what has been helping people cut down on waste? Using energy-efficient light bulbs. In other words, the act of using energy-efficient light bulbs, which is a noun.
