The significance of Hubble constant is:

Correct option is D

The correct answer is (d) It measures the rate of expansion of the universe

Explanation:
• The Hubble Constant ($H_0$) is the unit of measurement used to describe the expansion of the universe.
• It relates the distance of a galaxy to its recessional velocity: $v = H_0 d$.
• The reciprocal of the Hubble constant gives an estimate of the age of the universe.

Information Booster:
• Named after astronomer Edwin Hubble, who observed that distant galaxies are moving away from us.
• Current estimates of $H_0$ are around 70 km/s/Mpc.

Additional Knowledge:
Luminosity (Option a)
• Usually determined by the Inverse Square Law or standard candles like Cepheid variables.
Mass of Black Holes (Option b)
• Calculated using the orbital motion of nearby stars or gravitational waves.
Age of stars (Option c)
• Estimated using Hertzsprung-Russell (H-R) diagrams and stellar evolution models.

So the correct answer is (d)