Select the option that can be used as a one-word substitute for the given group of words.

A problem that is so difficult that it cannot be answered

Insoluble

The correct option is insoluble. It means "impossible to solve." The other options are not correct substitutes.

• Impossible means "unable to be done or achieved."
• Terrible means "extremely bad or unpleasant."
• Solvable means "able to be solved."