Jupiter and the Sun have almost the same composition and density; why isn't Jupiter also a star? ...

Answer to q. 1

Jupiter does not have enough mass to generate the 10 million degree central core temperature to sustain the proton-proton chain necessary to become a star.

Answer to q. 2

B

Answer to q. 3

A protostar of insufficient mass (less than 8 of the Sun's mass) that does not get hot enough from gravitational contraction to reach 10 million degrees and reach stage 7 as a stable main sequence star.

Answer to q. 4

As a gas cloud collapses, it breaks into smaller fragments. Each of these fragments in turn break into still smaller fragments. As the process continues, the small fragments become protostars and then stars.

Thanks for your help <3