(a) Similarities:
In both cases, the signal is initiated by receptors located at the specific sense organ.
In both cases, neurotransmitters are released and accepted by neurons to carry the impulse.
Differences:
The action of smelling the rose is voluntary whereas pulling the hand away is involuntary in nature.
While smelling the rose, the nerve impulse reaches the brain and back whereas on touching a thorn the nerve impulse travels only to the spinal cord and back.
(b) No, Involuntary actions occur with or without a stimulus whereas reflex actions always need a stimulus.