Final answer:
The isolated neutron star that must have had a binary companion is the pulsar inside a supernova remnant that pulses 30 times per second.
Step-by-step explanation:
In binary systems, neutron stars can have a binary companion. A binary companion is another star that orbits the neutron star. Among the given options, option a) is the correct answer as a pulsar inside a supernova remnant is likely to have had a binary companion. This is because the highly magnetic, rapidly rotating neutron star that forms a pulsar with a regular pulsing pattern could not have achieved such a rapid spin by the birth of the neutron star alone.