Final answer:
The stellar life cycle is a cycle because the death of stars leads to the redistribution of materials that will eventually form new stars, such as in the explosive event of supernovas which eject material back into space. This material can later coalesce due to gravity, forming new stars and continuing the cycle of stellar evolution.
Step-by-step explanation:
The life cycle of stars is considered a cycle because of the process by which stars are born, live their lives, and die, only to contribute material back into the cosmos that can lead to the birth of new stars. This is best illustrated by option c: 'Supernovas redistribute dust and gas which eventually forms nebulae.' When stars explode as supernovae, they expel their material into space, seeding the galaxy with the raw materials needed for new star formation. Over time, these ejected materials can coalesce due to gravity to form new stars, thus continuing the stellar cycle.
The gravitational collapse and nuclear fusion commencement in stellar nebulae begins the life of a star. Throughout their lives, stars undergo various changes dictated by their mass, with more massive stars living shorter lives. When their nuclear fuel is spent, stars die in different ways. Massive stars may explode in supernovas, creating neutron stars or black holes, and distributing material throughout the galaxy. Smaller stars, like our Sun, will shed their outer layers and create planetary nebulae. The remaining core becomes a white dwarf. These end-of-life events contribute to the stellar cycle by returning material to the interstellar medium, making it available for new stellar genesis.