What causes clouds of dust and gas to form a protostar?

(1) magnetism (3) expansion of matter

(2) gravitational attraction (4) cosmic background radiation

I believe that would be 2, gravitational attraction. Basically, one mass attracts smaller masses, until it becomes a protostar (Or planet, or anything else where this applies.).

The gravitational attraction causes clouds of dust and gas to form a protostar.

Explanation:

Stars are born inside the clouds of dirt and scattered throughout most galaxies. a well-recognized example of like a cloud is that the Orion Nebula. Turbulence deep inside these clouds offers rise to knots with a decent mass that the gas and mud will begin to collapse beneath its own attractive force. because the cloud collapses, the fabric at the middle begins to heat up. called a protostar, it's this hot core at the center of the collapsing cloud that may sooner or later become a star. Three-dimensional pc models of star formation predict that the spinning clouds of collapsing gas and mud might cut up into 2 or 3 blobs.