Answer:
Neutron Star
Step-by-step explanation:
"If the remaining core contains about 1.4 solar masses within a 20 km radius, it can become a neutron star."
If you are trying to find what a core will become with 3 or more solar masses, you are on the wrong question.
Neutron star is the correct answer for a core of a supernova containing about 1 solar mass.