Final answer:
Astronomers think neutron stars form from the collapse of massive stars during a supernova explosion, leaving behind incredibly dense stellar remnants. Observations such as pulsars and neutrinos from supernovae support this theory. Therefore the correct answer is d) Collapse of massive stars.
Step-by-step explanation:
Astronomers believe that neutron stars were formed in supernova explosions primarily due to the collapse of massive stars. During a supernova, the core of a very massive star undergoes gravitational collapse. This collapse is so intense that the star's outer layers explode, ejecting matter into space and leaving behind an extremely dense remnant composed almost entirely of neutrons called a neutron star.
This process occurs after a massive star has burned through its nuclear fuel, leading to a core comprised of iron, which does not generate energy through fusion. Eventually, the core exceeds the Chandrasekhar limit, leading to its collapse and rebound, creating a type II supernova explosion that results in the formation of a neutron star.
These neutron stars are the densest-known stars in the universe, with densities comparable to that of an atomic nucleus.
This understanding is supported by theoretical calculations and observations, such as the detection of neutrinos from a core collapse, as well as the characteristics of pulsars, which are rapidly rotating neutron stars that emit periodic pulses of radiation.