The correct option is
B Rain
Homophones are words that have the same or similar pronunciations, but different meanings. 'Reign' and 'Reign' are homophones.
Options A and B: 'Rain' is the water that falls from the sky, due to a process of evaporation and condensation.
'Reign', on the other hand, is the period through which a sovereign (ruler) occupies the throne.
'Rain' is more relevant to the given sentence, as it completes it in meaning.
Hence option B is correct and A is incorrect.
Options C and D are incorrect for the same: C cannot be correct because A is incorrect, and D cannot be correct because B is correct.