Final answer:
The coma and tail of a comet increase in size as the comet approaches and orbits around the Sun. The sublimation of comet material due to solar radiation creates a larger and more pronounced coma and tail.
Step-by-step explanation:
As comets approach and move around the Sun, the coma and the tail increase in size. The correct answer to the question is d) Increase. Comets are made of icy and dusty matter, and when they come near the Sun, some of their material vaporizes, creating a large head of tenuous gas and a tail. This tail is formed by the sunlight exerting a repulsive force, driving particles away from the head. Since the orbits of comets are highly elongated and have high eccentricity, they spend most of their time far from the Sun and have variable speeds, accelerating as they approach perihelion (closest point to the Sun) and decelerating as they move away. This change in distance from the Sun and speed leads to variations in the size of the comet's coma and tail, which increase significantly as the comet gets closer to the Sun due to increased solar radiation and the subsequent vaporization of comet material.