Earth is the only planet known to have
an atmosphere
a lithosphere
a biosphere
a cryosphere

A biosphere

Explanation:

Biosphere refers to the zone of life on a planet. It includes all living elements of a planet. It is also known as Eco-sphere and includes the interaction of all living being with their surrounding.

Of all the planets and exo-planets, Earth is the planet known to have life and thus biosphere.

Whereas atmosphere, lithosphere and cryosphere can be found on other planets as well.

Atmosphere: Layer of air and gases surrounding a planet or any celestial object. (Except Mercury, all the planets of solar system have atmosphere)

Lithosphere: it refers to the rigid outer part of the planet which includes crust and mantle. (All the rocky planets have lithosphere)

Cryosphere: portion of the planets where water is in ice form. (Many exoplanets have been discovered having cryosphere)

i am pretty sure it is c because when people are closer together, they cannot move as freely. picture being in a mostly full elevator, for instance. the people are right next to you, and you can barely move.

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it