If a hormone is released into the bloodstream and therefore comes into contact with many cells, what determines which cells in the body respond to the hormone?

THE CELL RECEPTOR DETERMINES WHICH CELL RESPOND TO AN HORMONE.

Explanation:

In the endocrine system, the hormones are usually released into the blood stream from where they traveled to their target organs. Even though the hormones meet a lot of cells on their way, they do not bind with just any cell. Hormones bind specifically to cells that have receptors that is meant specifically for the hormone. The cell receptor enables the binding together of the hormone and the cell membrane or the cell surface. After the binding, the hormone then elicit the needed response in the cell.  

well for one plants are always the producer, so a is the answer. a wolf is more of the consumer(predator) and the deer is the prey so i know those two are out of the question. also mushrooms are fungi/fungus so a is the producer.