Question

current observations of a new star forming support the nebular theory because they show a disk - shaped plane surrounding the star, which may eventually form (dropdown options: a protostar, a protosun, protonebula, protoplanets).

Explanation:

The nebular theory describes the formation of the solar system. A disk - shaped plane (protoplanetary disk) around a forming star contains material that can accrete to form protoplanets. A protostar is the early stage of the star itself, a protosun is a specific type of protostar (for our Sun's formation), and a protonebula is not the correct term for what forms from the disk. So the disk - shaped plane around the star can form protoplanets.

Answer:

protoplanets