Final answer:
None of the options will produce an agonistic effect if all post-synaptic receptors are blocked, because agonistic effects depend on the activation of these receptors. Precursors, reuptake inhibitors, and autoreceptor-induced release all rely on unblocked receptors to work.
Step-by-step explanation:
If you first block all of your post-synaptic receptors, none of the options provided will produce an agonistic effect. An agonist is a drug that activates certain receptors in the brain. However, if all post-synaptic receptors are blocked, administering a precursor will not help because the precursor's eventual product (the neurotransmitter) cannot act on any receptors since they are blocked. Similarly, blocking reuptake would increase the level of neurotransmitter in the synaptic cleft, but this would not lead to an agonistic effect because the receptors that would normally respond to this neurotransmitter are not functional. Autoreceptors stimulating the release of vesicles would also fail to create a response because the released neurotransmitter would find no available receptors to activate.
The agonistic effect entirely depends on the activation of post-synaptic receptors. With all receptors blocked, the pathways for an agonistic response are essentially nullified, and neither increased neurotransmitter levels due to inhibited reuptake nor elevated release from vesicles induced by autoreceptors will overcome this blockade.