What are you talking about exactly ? Here the original sentence is singular ("elle").
That's why it was translated as "Does she love my book ?".
If you're thinking that the exercise should also accept "Est-ce qu'elles aiment mon livre?" (plural), you're wrong.
In the plural sentence, when we pronounce it, we make the "liaison" between the "s" of "elles" and the "ai" of "aiment".
The final sound is a "z" sound, like so :
"Est-ce qu'elles Z aiment mon livre ?"
Try both sentences on Google Translate, you'll notice the difference by ear.
Am I the only one who couldn't tell if the speaker said "il" or "elle"? I had the rest of the words correct, but I seriously cannot hear the difference between sexes and plurals, when listening to the exercise. The narrator needs to pump up her pronunciation for us beginners who hate losing hearts over something so little