I actually think C is best. Here's my thinking:
Brain size doesn't necessarily correspond to animal intelligence. The animals with the largest brains aren't necessarily the smartest, and the animals with the smallest brains aren't necessarily the least intelligent. Bees and mice are super smart, for instance, and I've met plenty of big dogs with big brains that are super not-smart, so I'd say A is out. Also humans. Lots and lots of not-smart humans.
I think B is wrong because play and intelligence aren't the same thing. Same goes for D. Preference for food is a kind of instinctual thing more than an intelligence thing.
But greyhounds running around a track are trained to do so. They're trained to follow a kind of stuffed animal on a rail to guide them as they race around. These dogs totally know that they're in a race and that their job is to run along the track, to go faster than all the other dogs, and to win. THAT is totally intelligence.
