Select the most appropriate meaning of the given idiom.

A bolt from the blue

something that happens suddenly

The most appropriate meaning of the idiom "a bolt from the blue" is something that happens suddenly.

The other options are incorrect.

• "Something causing a head injury" does not mean something that happens suddenly. A head injury can happen suddenly, but it is not an idiom.
• "Something that is hotly debated" does not mean something that happens suddenly. A hotly debated topic can be discussed suddenly, but it is not an idiom.
• "A strange dream that wakes one up suddenly" does not mean something that happens suddenly. A strange dream can wake one up suddenly, but it is not an idiom.

The idiom "a bolt from the blue" comes from the idea of a lightning bolt coming out of the sky without warning. In the same way, a bolt from the blue is something that happens unexpectedly or without warning. For example, you might say that the news of the death was a bolt from the blue.