Nebula is a birthplace or stellar nursery where stars take birth. Gases condense to form a nebula. Then these gases start to collapse under their own gravity. This heats up the matter which is condensing which gives rise to a protostar. After the formation of protostar, if the mass is sufficient and temperature is high enough, nuclear fusion reaction kicks in at the core and starts to release energy and enters the main sequence.
Hence, the correct order of process leading up to the formation of main sequence star is: option c
