Question

an isolated white dwarf will eventually explode as a supernova when electron degeneracy pressure overcomes the gravitational force holding it together. continue to burn hydrogen into helium until it runs out of fuel in its core. blow off its outer layers and become a planetary nebula. continue to cool indefinitely.

Explanation:

An isolated white - dwarf has already exhausted its nuclear fuel. It has no more hydrogen to burn and will not explode as a supernova (supernovae occur under different conditions, like in a binary system for white - dwarfs or for massive stars at the end of their lives). It has already passed the stage of blowing off outer layers to form a planetary nebula. Since it has no nuclear reactions to generate heat, it will continue to cool indefinitely.

Answer:

continue to cool indefinitely.