Select the most appropriate meaning of the underlined idiom.

Some colleagues at a workplace are really hard nuts to crack.

An inflexible person who is hard to convince

The most appropriate meaning of the underlined idiom is an inflexible person who is hard to convince.

The idiom "hard nut to crack" means someone who is difficult to persuade or convince. It is often used to describe someone who is very stubborn or set in their ways.

In the sentence, the colleagues at the workplace are described as hard nuts to crack. This means that they are difficult to convince or persuade to change their minds. They may be very stubborn or set in their ways, and they may not be willing to listen to new ideas.

The other options are not correct.

• Hardcore traditions in a family: This is not the same as hard nuts to crack. Hardcore traditions are strong and unchanging, but they are not necessarily difficult to convince.
• Someone showing extra concern over little things: This is not the same as hard nuts to crack. Someone who is concerned about little things is not necessarily difficult to convince.
• An easy-goer who can be easily convinced: This is the opposite of hard nuts to crack. An easy-goer is someone who is easily persuaded or convinced.