Explanation:

The correct answer is option 'C', the Main Sequence Star.

A star goes through several stages in its life cycle, starting from its formation as a protostar to its eventual death as a white dwarf, neutron star, or black hole. The stage characterized by the fusion of hydrogen to form helium in the star's core is known as the Main Sequence.

Main Sequence Star:
- A main sequence star is a stable phase in the life cycle of a star, where it spends the majority of its lifetime.
- During this stage, a star is in a state of hydrostatic equilibrium, where the inward gravitational force is balanced by the outward pressure generated by nuclear fusion in its core.
- The main source of energy in a main sequence star is the fusion of hydrogen nuclei (protons) to form helium through a process called nuclear fusion.
- In the core of a main sequence star, high temperatures and pressures cause hydrogen atoms to collide and fuse together, releasing a tremendous amount of energy in the form of light and heat.
- This fusion process, known as hydrogen burning, occurs through a series of nuclear reactions called the proton-proton chain.
- The fusion of hydrogen to form helium in the core of a main sequence star provides the star with a stable source of energy, which keeps it shining brightly and maintains its gravitational equilibrium.
- The energy generated by nuclear fusion in the core is radiated outwards, providing the star with the necessary pressure to counteract the inward force of gravity.
- The mass of a star determines its position on the main sequence. More massive stars have higher core temperatures and pressures, allowing them to fuse hydrogen more rapidly and burn through their fuel supply at a faster rate.

Other Stages in the Life Cycle:
- Protostar: The initial stage of a star's formation, where a dense cloud of gas and dust collapses under its own gravity to form a hot, condensed object.
- Red Giant: A phase that occurs later in a star's life when it exhausts its hydrogen fuel in the core and starts burning helium. The star expands and becomes much larger and brighter.
- White Dwarf: The final stage for low to medium-mass stars, where the exhausted core collapses and the outer layers are expelled, leaving behind a dense, hot core composed mainly of carbon and oxygen.

In conclusion, the stage characterized by the fusion of hydrogen to form helium in the core of a star is the Main Sequence.