D is the best answer. This question asks you to identify which compound, according to Snyder et al, does NOT bind to specific receptors in the brain. Phosphodiesterase, identified as an “enzyme that breaks down the chemical called cyclic AMP” (lines 40-42), is the only compound that is not identified as one that binds to specific receptors in the brain. All the other substances named bind to specific receptors in the brain and this fact has been mentioned in different parts of the passage - IBMX in line 9 of paragraph 3, Caffeine in line 2 of paragraph 1, Adenosine in line 3 of paragraph 1 and Theophylline in line 6 of paragraph 3.