The virus which causes smallpox is -

Correct option is D

Sol. Smallpox is caused by the Variola virus, a member of the Poxvirus family. The disease was highly contagious and often fatal, but it was eradicated globally in 1980 due to a successful vaccination campaign led by WHO.
Explanation of Each Option:
· Varicella: Incorrect; causes chickenpox, a different viral disease.
· Rubula: Incorrect; not related to smallpox; it refers to mumps virus.
· Influenza: Incorrect; causes flu, which is a respiratory illness.
· Variola: Correct answer; the causative agent of smallpox.