Correct Answer: when

Solution : The correct answer is option 2

Explanation:

- A conjunction is a word that connects other words, phrases, or sentences.
- Correlatives conjunctions are are used in pairs.

These are: no sooner...than, scarcely...when, both...and, although...yet, not only...but also, neither...nor, and so on.

- As an example, he is fluent in both English and French.
- As an example, the teacher had barely seen the pupil when he began studying.
- 'Hardly' and 'when' make a correlative pair.
- So the correct word is 'when'.
Correct sentence:You would hardly have known the young prince when the time came for him to appear before his grandfather.