The nebular hypothesis of the formation of the solar system assumes that the material that became the solar system began as a large spherical cloud of gas and dust, rotating slowly. As the solar system formed, most of this material was transformed into a compact, flattened disk, rotating more rapidly. What is the explanation for this change in shape and rate of rotation?

Then What process heated the early solar nebula as it slowly condensed toward a central protosun? I. e. why was the temperature rising in the center of the contracting nebula?

a) conservation of the angular momentum

b) As a consequence of the interaction between particles.

Explanation:

A star is formed in a molecular cloud of gas and dust, mainly composed of hydrogen and helium. The Nebular Theory establishes, for the formation of the solar system, that the cloud starts to collapse under its own gravity when it receives a shock wave from a near event, for example, a supernova explosion. That results in the cloud breaking in small pieces, and those pieces constitute a possible future star.

Then it begins to accrete and rotate as a consequence of the angular momentum. In the center of that disk when it reaches the necessary temperature and pressure a protostar will born.

However, as the solar nebular condenses in the center due to its own gravity, the density increases, allowing more collisions between the particles that are in the nebula (atoms, free protons, etc), so the pressure rises and the temperature increases.          

there are no contact forces mentioned on the list of choices that you've provided.