If the core of a supernova contains about one solar mass, the core will become a
protostar
red dwarf
white dwarf
neutron star
black hole

Neutron Star

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.

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.