Terjemahan:They are ducks.
For someone who learn Indonesian using this course:
It's odd to say "Mereka adalah bebek-bebek" for daily conversation. We will use "Itu (adalah) bebek".
Note: 1. We use "mereka" for people not for animals. We will use for animals if they are considered as people (as in fiction).
Itu = those (or that) or they (usually for things or animals)
Adalah is optional.
We don't have to repeat bebek to make it plural. But if you repeat it, it's okay.