B. Gravity compresses gases at the center of a solar nebula until temperatures are high enough for nuclear fusion to occur.
Explanation:
Stars are born in stellar nurseries known as nebula. Sun, the star at the center of the solar system was born in the solar nebula.
Nebula is a molecular cloud of dust and gases. Due to gravity, this cloud collapses under its own weight and condenses to form the core. It gathers more mass till the temperature of the core rises enough to start the nuclear fusion reaction. A star is born then.
