The correct answer is C. when.
The sentence “Hardly had he arrived when it started raining” is an example of a complex sentence with a past perfect construction. The past perfect construction is used to describe an event that happened before another event in the past. In this case, the event that happened before the other event is “he arrived.” The event that happened after “he arrived” is “it started raining.”
The other options are incorrect because they do not make sense in the context of the sentence. The word “after” would suggest that “he arrived” and “it started raining” happened at the same time. The word “than” would suggest that “he arrived” happened after “it started raining.” The word “before” would suggest that “he arrived” happened before “it started raining.”
Here is a diagram that shows the structure of the sentence:
[Past perfect] [when] [simple past]
[Hardly had he arrived] [when] [it started raining]
The past perfect construction is in bold, and the simple past construction is in italics.
I hope this explanation is helpful!