Question

which event signals the birth of a star?
a core shrinks and explodes in a supernova.
a protostar collapses, becoming dense and hot.
extreme heat and pressure causes nuclear fusion.
a mass of gas and dust grows and forms a protostar.

Explanation:

The birth of a star occurs when nuclear - fusion reactions start in its core. Extreme heat and pressure in the core of a collapsing cloud of gas and dust (protostar) initiate nuclear fusion of hydrogen into helium, marking the start of a star's life. A supernova is related to a star's death, a protostar collapsing further to become dense and hot is a stage before nuclear fusion starts, and the formation of a protostar is an earlier step in the star - formation process.

Answer:

C. Extreme heat and pressure causes nuclear fusion.