The correct answer is: A. Nonconductor
A dielectric is an insulator that can be polarized by an electric field. This means that the positive and negative charges in the dielectric will be slightly separated when an electric field is applied. This separation of charge creates an electric field of its own, which opposes the applied field. This is why dielectrics are used in capacitors to store charge.
Option B, instructive, is incorrect because dielectrics are not typically used to instruct people.
Option C, constructive, is incorrect because dielectrics do not typically create or build things.
Option D, destructive, is incorrect because dielectrics do not typically damage or destroy things.