Choose the most appropriate verb form to fill in the blank in the fol…

The correct answer is Option (4) → was writing

Past continuous with the simple past:

• Two past actions joined by when:
  • The short action that happened at one moment takes the simple past.
  • The longer action already going on at that moment takes the past continuous (was/were + verb-ing).
• This sentence: the Principal entered (short action, simple past). The student had started writing before he came in and was still writing, so "a student was writing on the blackboard".
  Complete sentence: When the Principal entered the class, a student was writing on the blackboard.

Why the Other Options are Incorrect?

Option 1: wrote — Incorrect.

• The simple past shows a single completed action.
• "When the Principal entered, a student wrote" would mean the student started writing after he came in. It does not show an action already in progress.
  Example: He wrote a letter to his friend yesterday.

Option 2: writes — Incorrect. The simple present does not match the past time set by "entered".
Example: She writes in her diary every night.

Option 3: is writing — Incorrect. The present continuous is for an action going on now. It cannot be used with a past clause like "when the Principal entered".
Example: Look, Rahul is writing on the board right now.

Contrast: the action already going on takes the past continuous ("was writing"). The short action that breaks in takes the simple past ("entered").