The function of a capacitor in an electric circuit is to?
a. measure the amount of current in the circuit.
b. increase circuit power
.c. allow current flow between its plates.
d. store electric charges.

Well let's see:

A). No.  A capacitor doesn't measure anything.

B). No.  The power delivered to the circuit is determined by
              the battery or power supply and all the things in
              the circuit that dissipate energy.  A capacitor doesn't
              do any of these things.

C). No.  If any current actually flows between its plates,
             the capacitor is shot and can't do its job, and
             must be replaced.

D). Yes. A capacitor stores charges on its plates, and
              electrical energy in the field between its plates.
   

The answer is that they are composed mostly of liquid water.