Select the most appropriate meaning of the given idiom.

To be in the dark

to not know something others are aware of

The most appropriate meaning of the idiom "to be in the dark" is to not know something others are aware of.

The other options are incorrect.

• "To always be in a bad mood about darkness" does not mean to not know something others are aware of. Being in a bad mood about darkness is a feeling, while not knowing something is a state of being.
• "To learn to face dark places without fear" does not mean to not know something others are aware of. Learning to face dark places without fear is a process, while not knowing something is a state of being.
• "To be afraid of darkness and refuse to go out at night" does not mean to not know something others are aware of. Being afraid of darkness and refusing to go out at night is a phobia, while not knowing something is a state of being.

The idiom "to be in the dark" comes from the idea of being in a dark room, where you cannot see anything. In the same way, someone who is in the dark about something does not know anything about it. For example, you might say that a student is in the dark about the exam questions.