Choose the most appropriate verb form to fill in the blank in the following sentence:-

When the Principal entered the class, a student _____ on the blackboard.

was writing

The correct answer is Option (4) → was writing

The sentence describes an action that was in progress in the past when another, single action interrupted it or occurred simultaneously.

• The action of the Principal entering the class ("entered" - Simple Past) is the shorter, completed action.

• The action of the student writing ("was writing" - Past Continuous) is the longer action that was ongoing when the Principal entered.

The structure is :When + Simple Past (interrupting action), + Past Continuous (ongoing action).

Completed sentence: "When the Principal entered the class, a student was writing on the blackboard."