Select the most appropriate meaning of the underlined idiom in the given sentence.

This problem is a hard nut to crack, it will take longer than they imagined.

is difficult to solve

The most appropriate meaning of the underlined idiom in the given sentence is is difficult to solve. The idiom "hard nut to crack" means a problem that is difficult to solve.

The other options are not as appropriate.

• Needs a lot of work means that something requires a lot of effort or time to complete.
• Is not interesting enough means that something is not stimulating or engaging.
• Involves breaking nuts means that something requires the use of force or violence.