What is the celestial body that a comet nears when its tail forms?

When a comet approaches the Sun, the heat and solar radiation create conditions that lead to the formation of its tail. As the comet gets closer to the Sun, the ice inside it begins to vaporize, and the dust and gas are pushed away by the solar wind, creating the characteristic tail that we see. This tail always points away from the Sun, regardless of the comet's trajectory.

The other options do not account for this fundamental aspect of comet behavior. The Earth, Jupiter, and Mercury do not play a direct role in the formation of a comet's tail, as the tail's formation is closely linked to the proximity of the Sun, which provides the necessary energy and solar wind to evoke these changes in the comet.