The correct answer to the question is option 2, gravitational attraction.

A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. This powerful gravitational force is created by the collapse of a massive star.

Option 1, nuclear fission, is the process of splitting an atomic nucleus into smaller parts. While nuclear reactions may occur within a black hole, they are not the cause of a black hole`s formation.

Option 3, nuclear fusion, is the process of combining atomic nuclei to form a heavier nucleus. Again, while nuclear reactions may occur within a black hole, they are not the cause of its formation.

Option 4, the collision of asteroids, is unrelated to the formation of black holes. While collisions between astronomical objects can occur in space, they do not directly lead to the creation of a black hole.

In summary, the correct answer is option 2 because the formation of a black hole is due to the gravitational attraction caused by the collapse of a massive star.