To replace the underlined phrase, the correct alternative will be:
On hearing the sad news, he fainted.

The correct answer is '2' i.e. When he heard .

Key Points

• The sentence "On hearing the sad news, he fainted." implies that the action of fainting occurred immediately after hearing the sad news.
• The most appropriate option to replace "On hearing" is "When he heard," as it maintains the correct sequence of events.
• Option 1 ("Just before he heard") is incorrect because it conveys a meaning of time before hearing, which is not what the sentence implies.
• Option 3 ("When he had heard") is incorrect because the use of the past perfect tense (had heard) is unnecessary here; the simple past tense "heard" suffices.
• Option 4 ("No sooner he heard") is incorrect because "no sooner" requires the auxiliary verb "did" (e.g., "No sooner did he hear").

Correct sentence: When he heard the sad news, he fainted.

Additional Information

• Use of "When":
  • "When" is used to indicate the time of an action, especially when one action happens immediately after the other.
  • Example: When the bell rang, the students left the classroom.
• Use of "No sooner":
  • "No sooner" is used to describe two actions that occur in quick succession. It is often followed by the auxiliary verb "did."
  • Example: No sooner did he step outside than it started raining.