Which is not a characteristic of an NP$-$complete problem?

Question $3$ options:

No efficient algorithm has been found to solve an NP$-$complete problem.

An efficient algorithm to solve an NP$-$complete problem may be possible.

If an NP$-$complete problem has an efficient solution, then all NP$-$complete problems will have an efficient solution.

All NP$-$complete problems can be solved efficiently.