Explanation:

- To switch off the light: This option is unrelated to the idiom "Be off," as it directly refers to turning off electrical lights or appliances.

- To not eat anything: This doesn't fit well with "Be off" as an idiom unless used in a specific context indicating dietary choices or abstention.

- To be in a bad mood: While moods can be described in various ways, this option doesn't align with the common use of the idiom "Be off."

- To go away: This is the most appropriate meaning of "Be off." It often implies leaving a place or departing from somewhere.