Select the option that is grammatically correct:

The correct answer is Option (4) → Should the lift stop working, please use the staircase near the canteen.

A condition without “if”:

• The sentence is a shorter way of saying: “If the lift stops working, please use the staircase near the canteen.”
• “If” is dropped, and the sentence begins with Should instead.
• The order changes: Should + the subject (the lift) + the base verb (stop, with no -s).
• So “If the lift stops” becomes “Should the lift stop”. The sentence is correct.

Why the Other Options are Incorrect?

Option 1: Little the new students knew that the warden was watching them from the balcony. — Incorrect. When a sentence starts with “Little” in the sense of “not at all”, it takes “did” + subject + base verb. “Little the new students knew” is wrong.
Corrected: Little did the new students know that the warden was watching them from the balcony.

Option 2: The reason the bus was late is because a tree had fallen on the highway. — Incorrect. After “The reason ... is”, we use “that”. “Because” only repeats the idea of “reason”.
Corrected: The reason the bus was late is that a tree had fallen on the highway.

Option 3: Ravi has read the two first chapters of his history book so far. — Incorrect. Words like “first” and “last” come before the counting number. We say “the first two”, not “the two first”.
Corrected: Ravi has read the first two chapters of his history book so far.