Post-synaptic membrane:
The post-synaptic membrane is a specialized region of the neuron that receives signals from the pre-synaptic neuron. It contains various structures that are essential for signal transmission and integration.

Receptors:
Receptors are proteins that are embedded in the post-synaptic membrane. These receptors are specific to neurotransmitters released by the pre-synaptic neuron. When a neurotransmitter binds to its corresponding receptor, it triggers a series of biochemical events that lead to the generation of an electrical signal in the post-synaptic neuron. These receptors play a crucial role in the transmission of signals across the synapse.

Synaptic knobs:
Synaptic knobs, also known as synaptic terminals, are small structures at the end of the pre-synaptic neuron. They contain synaptic vesicles filled with neurotransmitters. Synaptic knobs are not present on the post-synaptic membrane. They are responsible for the release of neurotransmitters into the synapse.

Synaptic vesicles:
Synaptic vesicles are small membrane-bound sacs found within the synaptic knobs of the pre-synaptic neuron. These vesicles contain neurotransmitters, which are released into the synapse in response to an action potential. Synaptic vesicles are not present on the post-synaptic membrane.

Schwan cells:
Schwann cells are a type of glial cell that forms the myelin sheath around axons in the peripheral nervous system. They are not directly involved in the post-synaptic membrane and are not present on it.

Therefore, the correct option is 'C' - Receptors. Receptors are the key structures present on the post-synaptic membrane that play a pivotal role in signal transmission and integration. They bind to neurotransmitters released by the pre-synaptic neuron and initiate the generation of an electrical signal in the post-synaptic neuron.