What type of electrical component consists of two or more conductive surfaces separated by an insulator?*
Well, I guess I gave it away in the post title. Yes, the answer is D. Capacitor. But what exactly is going on here?
A capacitor is a device that has the “capacity” (get it?) to store energy in an electric field. It does this by collecting charge on one of two plates, separated by an insulator called a dielectric. The charge couples with the conductor on the other side and creates an electric field inside the dielectric. (See the image above.) The most obvious type of this is the typical ceramic disc style capacitor. It’s easy to visualize the construction that way. A “rolled” foil regular or electrolytic capacitor is built the same way, except the whole 3-layer construct is rolled into a nice tight package.
An “air” variable capacitor (see above) like ones that were used in early radios for tuning, simply use the air in between the fins as the dielectric. You’ll still find these in manual antenna tuners, for example.
Looking at the other possible answers, though: A potentiometer is simply a type of variable resistor. Neither is constructed in this manner. An oscillator is a type of circuit, not a component therein.