Choose the sentence that has the correct preposition in it:

The correct answer is Option (4) → Radha will not be able to reach within half an hour.

Within means before the end of a stated period of time. "Within half an hour" correctly gives the time limit by which Radha cannot arrive.
Example: The pizza will be delivered within thirty minutes.

Why the Other Options are Incorrect?

Option 1: They will return with fifteen minutes. — Incorrect. "With" shows company or means, not a time limit; the sentence needs "within fifteen minutes" (or "in fifteen minutes").
Corrected: They will return within fifteen minutes.

Option 2: The students came onto the classroom. — Incorrect. "Onto" shows movement to a surface (onto the stage); entering an enclosed space needs "into the classroom".
Corrected: The students came into the classroom.

Option 3: Susheel works on the office every weekend. — Incorrect. A place of work takes "at" or "in": "works at the office". (One works on a task, not on a place.)
Corrected: Susheel works at the office every weekend.