The guard spent all the night pacing … .

Analyze the question: The sentence needs a fixed expression that describes walking back and forth. "Pacing" means walking repeatedly in one direction and then the other, usually because of restlessness or while on watch.

Explain the concept: English has an idiomatic phrase for this: "to and fro". It means back and forth. "Fro" is an old word meaning "away" or "back" (it is related to "from"), and it survives today only in this phrase. Idioms like this have a fixed word order, so you cannot rearrange them.

Solution:

• A. To and fro: This is the correct, standard idiom.
• B. From and to: Not an English idiom.
• C. Fro and to: The order is reversed, so it is wrong.
• D. To and from: This is used for travel between two places (e.g., "travelling to and from school"), not for pacing. It also needs an object, such as "to and from the gate".

Complete sentence: "The guard spent all the night pacing to and fro."

Tip: Remember the pairing "to and fro" like "back and forth." Both phrases describe movement in two directions. If you see "fro" in an option, it almost always belongs in "to and fro." Also note that "to and from" is used for journeys between two places, while "to and fro" is for movement back and forth.