Explanation:

- Here's what each part of the sentence implies:

- P: Indicates a promise made to return home early.

- Q: Describes a phone call made to the mother after reaching the office.

- R: Suggests that the speaker was unable to fulfill a promise.

- S: Clarifies the intent of the phone call, which was to inform the mother.

- Let's examine the options:

- Option 1 (PQRS): Starts with reaching home early, which doesn't make sense without context.

- Option 2 (RPQS): Begins with the inability to do something, which also lacks context.

- Option 3 (QSRP): Begins with calling the mother, providing context to follow up with more information.

- Option 4 (SQPR): Starts with informing without context; doesn't construct a logical sentence.

- Option 5 (None of these): Irrelevant in this case as option 3 is a valid sentence.

- Correct Sentence: "When I reached my office, I phoned my mother to tell her that I wouldn't be able to get home early as I had promised."