I would say the best answer is A, he hides behind a tree. I was in between A and D, but one can run in a park.
B wouldn't work because him being dressed in ordinary street clothes doesn't show that he's afraid of something.
C wouldn't work because although yes, he was panting, that could mean that he's just tired; not necessarily, scared.
The man hiding behind the tree is the best evidence for the conclusion of him being afraid of something. Also in the passage it states, "....he moved behind a tree. Seeming not to notice my presence, he stood with his back against the trunk, panting."
