Yeah, mbwa is singular and plural. Most animal names belong to the so-called N-class and are identical in singular and plural. Iften, something else in the sentence helps you out.

Mbwa anakimbia. = The dog is running.
Mbwa wanakimbia. = The dogs are running.

You could also say:

Emilian anampenda mbwa. = Emilian loves the dog.
Emilian anawapenda mbwa. = Emilian loves the dogs.

Adding the object prefix to the verb makes it more likely to be interpreted as definite though, so the sentence with the simple anapenda is a bit better for a general statement.

Of course that is the correct answer. English grammar in this course is just totally appalling.

Natuurlijk is dat het goede antwoord. De Engelse grammatica in deze cursus is gewoon kut met peren (Warning: very colloquial Dutch :)