I just took the quiz on Edguenity and got a 100. If you want to double check my answer, it is in the first sentence of the audio. It says something like Elisa fue al mercado...
Being fluent in Spanish, I can tell you the answer is a. The option b is plural... You'd only choose it if they were talking about more than one puppy. But SINCE you're talking about a singular baby, the answer is a.
