What Might Have Made the Original Solar Nebula Begin to Contract

A) the shock wave from a nearby exploding star
B) the large amount of angular momentum in the nebula
C) interstellar magnetism generated by pulsars
D) the formation of our arm of the Milky Way
E) the Big Bang

A) the shock wave from a nearby exploding star


The shock waves from one or more nearby super nova are what gave the proto-solar nebula its angular momentum. Conservation of mass, energy AND momentum did the the rest. The laws of physics in this Universe are isotropic.

F) All objects have gravity, however little. There was a lot of dust and debris from the star's predecessor's explosion and other star's explosions floating around. Eventually, particles clump up due to gravity. Over a long period of time, bigger clumps. Eventually, the clump gets big enough for fusion and a star is born. (This takes a REALLY long time.)
This is also what's happening inside nebulae, space gas from past explosions.

Most likely a supernova shock wave due to the abundance of heavy elements in the solar system.

A).
. .

A) plus a good bit of gravity over a long time.