Here's why:
* Idioms are phrases that have a figurative meaning that is different from the literal meaning of the words.
* "Spread the word" means to tell many people about something, not literally spreading words around.
Let's compare it to the other options:
* Slang is informal language that is specific to a particular group or region. "Spread the word" is not specific to any particular group, and it's widely understood.
* Informal language is casual and relaxed. "Spread the word" can be used in both formal and informal settings.
Therefore, "spread the word" best fits the definition of an idiom.