The temperature of a superconductor is gradually lowered. at the critical temperature, how does the resistivity of the superconductor change?
it reaches a constant positive value
it suddenly drops to zero
it begins a gradual decrease
it experiences a spike to its highest value

Second Option: it suddenly drops to zero

Explanation:

Superconductors are resistance free materials that enable (allow) electricity conduction (i.e. electron transportation between atoms).

As the superconductor temperature is gradually lowered, eventually a Critical Temperature is reached, where the superconductor electrical resistivity suddenly drops to zero. Such drop is so fast, that can be described as a complete matter-phase transition. This is in contrast with typical conductors, where with gradually decreasing temperature there is also a gradually decreasing resistivity (which is also true at temperatures of near absolute zero).

Thus the correct answer is the Second Option: it suddenly drops to zero