Final answer:
A main-sequence star is hotter and brighter than it was as a protostar due to the start of nuclear fusion in its core, which significantly raises its temperature and luminosity.
Step-by-step explanation:
When comparing the surface temperature and luminosity of a protostar to that of the main-sequence star it becomes, we observe significant changes. Initially, protostars generate energy through gravitational contraction, which increases their temperature as they contract, but their overall luminosity is typically lower due to their large size and low density. Once nuclear fusion starts in the core, marking the beginning of the main-sequence phase, the star's temperature and luminosity both increase.
The correct answer to the question is B) A main-sequence star is hotter and brighter than it was as a protostar. This is because main-sequence stars have commenced nuclear fusion processes in their cores, which results in a major increase in both temperature and luminosity compared to their earlier protostellar phase.