Question

which object forms when a supergiant explodes?
a red giant
a protostar
a white dwarf
a neutron star

Explanation:

A supergiant's explosive death (a supernova) leaves behind a dense core. Red giants are evolved low-mass stars, protostars are pre-main sequence stars, white dwarfs are remnants of low/medium-mass stars. Only neutron stars form from supergiant explosions (for stars 8-20x the Sun's mass).

Answer:

a neutron star