Answer:
A. Scientists who study animals have found that mammals and birds dream while they sleep.
Step-by-step explanation:
It won't be B because the scientist have a good idea on what cat and dogs dream about.
It won't be C because the paragraph says that fish and reptiles don't dream.
It won't be D because they already have a good idea on what animals dream about.
That leaves A to be the correct answer in my opinion.
Hope this helps ^-^