The correct answer is option C – A virus is composed of a core of DNA or RNA. All viruses have a genetic makeup of either RNA or DNA. Viruses are classified based on either of these nucleic acids and whether they are a single-stranded or double-stranded nucleic acid. This core of RNA or DNA is surrounded by a protein coat called “capsid.” Option A is wrong because not all viruses have DNA.
Some have RNA as nucleic acid instead of DNA. Option B is also wrong because the building block of viruses is not cells. The building blocks of viruses are called virions. Option D is incorrect because viruses are not composed of nerve tissue. Viruses can affect nerve tissues, and some viruses become latent in nerve tissue.