The key characteristics of NP-complete problems are:
Their solutions cannot be found in polynomial time.
Their solutions, once provided, can be verified in polynomial time.
They are considered to be among the hardest problems in NP
Options A, B and D
NP-complete problems are a subset of NP problems that are believed not to be solvable in polynomial time but have the property that if there exists a polynomial-time algorithm for any one of them, then all problems in NP would also have polynomial-time solutions.
Therefore, they are considered challenging and among the most complex problems in computational theory.