Which planet is suitable for human habitation? Red giant? Mercury? Mars?

If you have to choose one of the three, there is no doubt that you choose Mars.

Red giant is a kind of celestial body whose star evolves into old age, causing internal helium fusion to expand violently, and its surface temperature decreases and turns red. Although the surface temperature is not too high, the temperature around 3000K is enough to decompose and ionize most molecules.

The limitations of mercury are similar. First, the daytime temperature is too high, which can exceed 300℃, enough to decompose most organic matter. Secondly, without atmospheric insulation, the temperature will drop sharply at night, reaching-100℃.

Although there are still many problems in moving to Mars (no oxygen, no water, no crops), it is much milder and more suitable than the first two.