According to present understanding, a nova is caused by _________.

A nova is indeed caused by hydrogen fusion on the surface of a white dwarf. In a binary star system, one component can evolve into a red giant while the other partner remains a white dwarf. If the white dwarf is close enough to its companion, it can accrete material, primarily hydrogen, from the outer layers of the red giant. As this hydrogen accumulates on the surface of the white dwarf, pressure and temperature increase until conditions become favorable for nuclear fusion to occur.

The sudden onset of hydrogen fusion leads to a thermonuclear explosion on the surface of the white dwarf, dramatically increasing its brightness for a brief period, which is observed as a nova. This event can recur multiple times as the white dwarf continues to collect hydrogen from its companion star.

The other options describe different astrophysical phenomena. Helium fusion in a red giant refers to the process that occurs later in the life cycle of a star, not related to nova events. Massive star collapse pertains to supernova remnants from massive stars that do not involve white dwarfs. Electron capture is a process that leads to the formation of neutron stars and is not involved in the mechanism that causes a nova. Understanding these distinctions reinforces why hydrogen fusion on a white dwarf is the correct answer.