The correct option is A Birth
Homophones are words that have the same or similar pronunciations, but different meanings. 'Birth' and 'berth' are homophones.
Options A and B:
'Birth' refers to the action of being born.
It relates to the given sentence that states the action of the 'son' being born.
'Berth' refers to a shelf like sleeping place, as on a ship or in a train.
This word does not fit into the sentence in meaning.
Hence option A is correct and B is incorrect.
Options C and D are incorrect for the same: C cannot be correct because B is incorrect, and D cannot be correct because A is correct.