I believe the correct answers are: post-synaptic receptors
and shape of the neurotransmitter.
In the lock-and-key analogy for neurotransmitters, the lock
refers to the post-synaptic receptors, whereas the key refers to the shape of
the neurotransmitter.
Analogy of key-and-lock refers to the binding to occur
between post-synaptic receptors and the shape of neurotransmitters, in which the
neurotransmitter and the receptor site must have perfectly complementing
shapes, as does a key that fits a lock.
