The correct option is
D Virus
A virus is a small infectious agent that replicates only inside the living cells of other organisms. Virus is made of the genetic material which is either DNA or RNA that carry genetic information, a protein coat called the capsid which surrounds and protects the genetic material and an envelope of lipids that surrounds the protein coat. Viruses do not contain enzymes or any other metabolic machinery so they use host machinery after infection. So, the viruses are called as intracellular obligate parasites.
So, the correct answer is 'Virus'.