The correct answer is They are true bipolar neurons.

- Olfactory receptor cells are unique among sensory neurons because they are true bipolar neurons.
- This means they have two distinct processes: one dendrite that extends to the surface of the nasal epithelium and ends in olfactory cilia, and a single long axon that projects centrally to the olfactory bulb of the brain.
- This distinctive bipolar structure allows them to directly detect odor molecules and transmit sensory signals.

- Other sensory neurons, like those in the dorsal root ganglia, are typically pseudounipolar neurons, where a single axon splits into two branches rather than having distinct dendrites and axons. Therefore, option 1 is incorrect.

- Regarding option 3, olfactory receptor cells do contain synaptic vesicles at their axon terminals because they release neurotransmitters to communicate with second-order neurons in the olfactory bulb.

- Option 4 is also incorrect because while olfactory receptor cells can regenerate, they do so from basal stem cells, not from supporting cells.

Reference: Gray's Anatomy, 41st Edition, Nervous System, Chapter 12 (Olfactory System), Pg. 875-877