All stars on the main sequence...

A) are in the process of forming

B) are mature stars that are fusing hydrogen into helium in their cores

C) are dead stars

D) have exhausted their nuclear fuel and are in the process of dying

B) are mature stars that are fusing hydrogen into helium in their cores

Main sequence stars are in hydrostatic equilibirium between gravity and pressure.

Mature stars are stars in which gravity is high enough to cause stellar nucleosynthesis. In other words, the pressure is enough to crete temperatures to cause fusion of hydrogen to helium, with the release of huge amounts of energy. So the answer should be B.
A is a protostar.
C could be white dwarf or a neutron star, depending on the size and composition of the star.
D could be the final phases of red giant or beginning of planetary nebulla.

A) false - these are not yet on the main sequence

B) true - this is the definition of a main sequence star. If a star runs out of fuel it will collapse. If it is burning anything else it will be a giant stat off the main sequence.

c) false - white dwarfs, neutron stars and black holes are not on the main sequence

D) false - all stars on the main sequence are fusing their hydrogen fuel.