Answer : 4. "would have seen"
Explanation :

• The correct tense to be applied in this sentence is “would have seen”

• The third conditional form of the sentence is used to discuss a hypothetical previous circumstance and its potential outcome.

• The past perfect tense of the “if-clause” in this sentence (had gone) denotes that the situation did not actually occur.

• The conditional perfect tense (would have seen), which denotes a hypothetical outcome in the past if the fictitious circumstance had occurred, is thus necessary for

the main phrase.

Example: I would have received a better grade on the exam if/ had studied harder.

• The speaker is imagining a scenario in which they didn’t put enough effort into their studies, which led to a poorer grade.

• Therefore, the complete sentence is: If he had gone to Agra, he would have seen the Taj Mahal.