Final answer:
Neutron stars' interiors are composed chiefly of neutrons in a superfluid state, as the conditions within are too extreme for any other state of matter to exist, such as a dense gas or a metallic fluid.
Step-by-step explanation:
The interior of a neutron star is an incredibly dense and exotic environment due to the overwhelming gravitational forces present. As the core of a massive star collapses during a supernova, protons and electrons combine to form an almost pure neutron composition. This results in a state that is believed to be a superfluid of neutrons at the inner layers, due to the quantum mechanical effects that arise under such extreme conditions. The crushing pressure can also support the presence of a crystalline lattice structure in the outer layers of the core.
A neutron star's interior is not a dense gas of neutrons, nor a metallic fluid of almost pure iron, as these states would not withstand the extreme densities reached inside a neutron star, which are comparable to the density of an atomic nucleus.
Therefore, the most accurate description of a neutron star's interior, from the provided options, would be that it consists of neutrons in a superfluid state. This state allows neutrons to flow without viscosity, which could never be achieved in a regular fluid or a dense gas.