Answer:
Option: B. the anti-Christian policies of the Vietnamese monarchy.
Step-by-step explanation:
France and Vietnam relations started in the 17th century when Jesuit Father Alexandre de Rhodes decided to spread Catholic Christianity in Asia. It was there the first time to establish missionary outside their nation. Soon afterwards it was seen that Christianity was not much welcome in Vietnamese monarchy. Few missionaries were killed, by seeing this France declared to attack and capture the city in the order of Emperor Napoleon III carried by Charles Rigault de Genouilly in 1858.