Smallpox is caused by the Variola virus, so option 2 is the correct answer. Rubeola virus, option 1, is responsible for causing measles, not smallpox. Varicella, option 3, refers to the virus that causes chickenpox and shingles. Myxovirus, option 4, is a broad term that includes various types of viruses, such as influenza and mumps, but it does not specifically cause smallpox. Therefore, options 1, 3, and 4 are incorrect. It is important to note that Smallpox has been eradicated globally since 1980, thanks to successful vaccination efforts.