Take a look at the sentence again:
When Simone and her husband discovered sixteen jars full of insects in their
daughter's room, they decided to get Maria
a dog, tolerating one four-legged animal seemed
better than having fifty six-legged "pets" in the house.
When Simone and her husband discovered sixteen jars full of insects in their daughter's room is a subordinate clause. A subordinate clause cannot stand alone as a complete sentence. This subordinate clause introduces the main clause that follows, they decided .... A comma is necessary to connect these two clauses.
Go back to the sentence to try again.