Detailed Explanation:

Answer: Option 4 — Speed of light is always same.

Astronomical distances are measured in light-years because the speed of light in a vacuum is a universal constant (approximately 299,792 km/s). This constant nature makes it a reliable and standardized unit for measuring the vast distances in space. A light-year represents the distance light travels in one year, providing a practical scale for cosmic distances.

❌ Statement A – Incorrect: Distances between stellar bodies do change due to orbital motion, stellar evolution, and galactic movements; this is not why light-years are used as a unit.

❌ Statement B – Incorrect: While gravity is fundamental, the gravitational effects of stellar bodies vary with mass and distance changes; this has no bearing on the choice of light-year as a distance unit.

❌ Statement C – Incorrect: Light does not always travel in a straight line; it bends around massive objects due to gravitational lensing, as predicted by Einstein's General Theory of Relativity.

✅ Statement D – Correct: The speed of light in a vacuum is constant at approximately 3 × 10⁸ m/s, making it an ideal standard for measuring astronomical distances.

📝 Short Notes: Astronomical Distance Measurements

• Light-Year: The distance light travels in one year in a vacuum (approximately 9.46 trillion kilometers or 5.88 trillion miles).
• Astronomical Unit (AU): The average distance from Earth to the Sun (approximately 150 million km); used for measuring distances within the solar system.
• Parsec: Another unit for astronomical distances, equal to about 3.26 light-years; commonly used by professional astronomers.
• Speed of Light (c): A fundamental physical constant equal to 299,792,458 meters per second in vacuum; remains constant regardless of the observer's motion or the source's motion.
• Gravitational Lensing: The bending of light around massive objects due to gravity, predicted by Einstein's General Theory of Relativity and observed around galaxies and black holes.