What's an Air Parcel?

All three answers are correct.

It's a little strange applying Newton's laws to something like the atmosphere, because we don't experience the atmosphere as a body, or even a collection of bodies: we experience it as a continuous fluid. Instead of maintaining its shape, parts of the atmosphere are continuously flowing and deforming. Yet, each individual "piece" of the atmosphere obey's Newton's laws.

Even the individual molecules obey Newton's laws pretty closely, but the motion of an individual air molecule is quite a bit different from the motion of the wind. Individual molecules are constantly vibrating and bouncing off each other, producing the effects that we call temperature and pressure. But if we are to understand what makes the wind blow, we must consider a mass of air large enough to include lots of molecules, but small enough that whole assemblage moves like a single object. Meteorologists call such an imaginary object an "air parcel".

I like to think of an air parcel as the air inside an imaginary balloon. I imagine the balloon as weightless, so it just floats there. Unless a gust of wind (or a playful human) comes along, the balloon will just stay in one place. But if you go up to it and tap it, it will drift off in that direction.