**Core Concept**
The capsid is a structural component of viruses, composed of proteins that form a protective layer around the viral genome. It plays a crucial role in the virus's ability to infect host cells and replicate.

**Why the Correct Answer is Right**
The capsid is indeed the protein coat that surrounds the viral nucleic acid, typically DNA or RNA. This protein shell is composed of multiple copies of one or more proteins, which self-assemble to form a stable structure. The capsid helps protect the viral genome from environmental stressors and facilitates the attachment and entry of the virus into host cells. The arrangement and structure of the capsid proteins can also influence the virus's ability to evade the host immune system.

**Why Each Wrong Option is Incorrect**
**Option A:** The extracellular infectious particle is actually the complete virus particle, including the capsid, nucleic acid, and any viral envelope. This option is incorrect because it describes the entire virus, not just the capsid.
**Option C:** The envelope around a virus is a lipid layer that surrounds the capsid in some viruses, but it is not the same as the capsid itself. This option is incorrect because it refers to a separate component of the virus.
**Option D:** This option is incorrect because one of the options above (B) accurately describes the capsid.

**Clinical Pearl / High-Yield Fact**
The structure of the capsid can influence the virus's ability to infect host cells and evade the immune system. Understanding the structure and composition of viral capsids is essential for the development of antiviral therapies and vaccines.

**✓ Correct Answer: B. Protein coat around nucleic acid**