Answer:
A. Dolphins are actually the smartest mammal (besides humans), but they are nearly impossible to keep as pets.
Step-by-step explanation:
Every statement, except for statement A, is somehow related to animals positively influencing people's mental health.
Statement D is the most relevant, as it tells about autistic children and the effect horses' company has on them.
Statements B and C aren't directly related to the topic as they do not mention autistic children, but they do tell about the positive effect animals have on people's anxiety level.
Statement A, however, only gives us interesting information about dolphins.