What Type of Signal Transduction Process Is First Initiated When

What type of signal transduction process is first initiated when an odorant binds with a receptor on a cilium of an olfactory receptor?

A) Ionotropic receptors lead to the synthesis of cyclic AMP.
B) G protein receptors lead to the synthesis of cyclic AMP.
C) Ca²⁺ gated chloride channels lead to the synthesis of cyclic AMP.
D) Ionotropic receptors lead to the activation of G proteins.
E) Cyclic AMP gated channels lead to the activation of G proteins.