In this question, the correct answer is option 3: Myanmar. Recently, a large number of monks in Myanmar conducted protest marches against the government. It is important to note that monks hold a significant role in Myanmar society, and their actions often have a strong impact on public discourse and opinion.

Option 1: Cambodia. While protests do occur in Cambodia, specifically led by activists and opposition groups, there is no recent significant event involving a large number of monks conducting protest marches against the government.

Option 2: Japan. There have been protests in Japan, but these have been led by various citizen groups and organizations, not specifically by monks. So, it is not the correct answer.

Option 4: Vietnam. Protests in Vietnam are not uncommon, especially in relation to issues such as human rights and land disputes. However, in recent times, there has been no reported incident of a large number of monks making protest marches against the government in Vietnam.

Therefore, the correct answer is option 3: Myanmar.