Final answer:
A star on the horizontal branch of the H-R diagram is in the red giant phase of its life cycle. It is not a main-sequence star, a white dwarf, or a protostar. Hence, the correct answer is option (b).
Step-by-step explanation:
If a star is on the horizontal branch of the H-R diagram, then the correct statement about the star's current stage of life is that it is in the red giant phase. The horizontal branch is a stage that follows the main sequence phase for stars of certain masses and precedes the ultimate stages of a star's life, such as becoming a white dwarf.
Main-sequence stars are located along the continuous and distinct band that cuts diagonally across the H-R diagram. They spend the majority of their lifetime in this phase, fusing hydrogen into helium in their cores. As main-sequence stars like the Sun consume their hydrogen fuel, they evolve off the main sequence, expanding and cooling to become red giants.