The sympathetic chain ganglia are a series of clusters of neuron cell bodies linked by bridges of
a. blood vessels
b. axons
c. myelin
d. dendrites

b. axons  

Explanation:

The Peripheral Nervous System consists of nerves and ganglia. Nerves are nothing more than bundles of parallelly arranged nerve fibers surrounded by connective tissue. Each of these fibers is formed by an axon and the sheaths surrounding it. The ganglia, in turn, are accumulations of neurons, which usually form spherical structures, are located outside the central nervous system and are also linked by axons bridges.