It is very stable and allows viruses to exist in water, air, and the ground.

Explanation

a. Nucleoproteins-Proteins bound to nucleic acids; important for genome function but not for environmental stability.b. Nucleocapsid-Viral genome plus capsid; protects the genome, but stability mainly comes from the capsid.c. Tail of virus-Found in some bacteriophages; used for attachment and injection, not for stability.d. Capsid (Correct Answer)-Protein shell protecting the viral genome; crucial for stability and survival in water, air, and soil.