Answer:
Option B, Gravity
Step-by-step explanation:
Due to gravity, the matter with in the interstellar cloud collate together to form star.
A star form when the interstellar gas or dust (molecular dust) comes closer to form dense part under the influence of their own gravity. The core of a gas cloud is dense and hence it collapse first to form clump and then the outer gas collide to form more clumps. These clumps then get collate and get converted into protostars and later turn into fully developed star.
Hence, option B is correct