Question

why does mercury have a very thin and temporary atmosphere?
the heat of the sun burns off any atmosphere as soon as it is formed
the planet rotates so quickly that its velocity expels atmospheric gasses as they condense
the mass of the planet is too small for an atmosphere to continuously stay around the planet
mercury lacks the gaseous elements (hydrogen, carbon dioxide, etc.) it takes to sustain an atmosphere

Explanation:

Mercury has a very thin and temporary atmosphere because its mass is too small. A planet's mass determines its gravitational pull. A stronger gravitational pull can hold gases (atmosphere) around the planet. Mercury's small mass means its gravitational pull is weak. So, gases that could form an atmosphere are not held tightly and can escape into space. The other options are incorrect. Mercury's rotation is slow (not fast as in option 2). While the sun's heat can affect, it's not the main reason (option 1). And Mercury does have some gaseous elements (option 4 is wrong).

Answer:

The mass of the planet is too small for an atmosphere to continuously stay around the planet.