The correct answer is option 'C', which states that the Hubble Constant describes the rate of the universe's expansion. The Hubble Constant is a fundamental parameter in cosmology that quantifies the rate at which the universe is expanding. Let's explore this concept in more detail.

1. What is the Hubble Constant?
The Hubble Constant, denoted by the symbol H₀, is a measure of the current rate of expansion of the universe. It is named after the American astronomer Edwin Hubble, who first discovered the expansion of the universe in the 1920s. The Hubble Constant is expressed in units of kilometers per second per megaparsec (km/s/Mpc).

2. The Expansion of the Universe:
Observations have revealed that galaxies are moving away from each other, and the space between them is expanding. This phenomenon is known as the expansion of the universe. The Hubble Constant quantifies the rate at which this expansion is occurring.

3. Hubble's Law:
Hubble's Law, formulated by Edwin Hubble, mathematically describes the relationship between the recessional velocity of a galaxy and its distance from us. It can be expressed as v = H₀d, where v is the velocity of recession, d is the distance to the galaxy, and H₀ is the Hubble Constant.

4. Measuring the Hubble Constant:
Determining the precise value of the Hubble Constant is a complex task and has been the subject of ongoing research. Scientists employ various techniques, such as measuring the redshift of distant galaxies, analyzing the cosmic microwave background radiation, and studying the movements of stars in nearby galaxies, to estimate the Hubble Constant.

5. Implications of the Hubble Constant:
The value of the Hubble Constant has significant implications for understanding the age, size, and future fate of the universe. It provides insights into the expansion history of the universe and helps in determining the composition and energy content of the cosmos.

In conclusion, the Hubble Constant represents the rate at which the universe is expanding. It is a fundamental parameter in cosmology and plays a crucial role in our understanding of the nature and evolution of the universe.