Question

which is the final event that occurs when a star is forming?
the outer layer of the star expands.
gravity pulls gas and dust together.
a black hole forms as the core collapses.
nuclear fusion begins under high pressure.

Explanation:

When a star is forming, gravity first pulls gas and dust together. As the core gets denser and hotter, nuclear fusion (specifically hydrogen - to - helium fusion) begins under high pressure. This is the final event in the star - forming process. The expansion of the outer layer is part of the later stages (like in red - giant formation for some stars), and black - hole formation is a result of the end - stage of very massive stars (not the final event in star formation).

Answer:

Nuclear fusion begins under high pressure.