When does a protostar become a true star?
A) when the star is 1 million years old
B) when the central temperature reaches 1 million Kelvin
C) when nuclear fusion begins in the core
D) when the thermal energy becomes trapped in the center
E) when the stellar winds and jets blow away the surrounding material

C) when nuclear fusion begins in the core

Explanation:

The protostar is a main-sequence star that has a core temperature that exceeds abut 10 million K and is needed for the hydrogen to operate efficiently and the length of this process depends on nature and the stellar mass, as the huge massive star have a greater and bigger core and fusion takes fast. A protostar is also called as primitive star and represents the earliest phases of the process of the stellar evolution and takes place after it leaves form the main sequence star.

Here is a picture  of the net. its 4 squares in a row diagonal and on the third draw one square aove and under

I'm pretty sure it's c. extremely gradual but not 100% sure though.