What is a mature virus particle containing a nucleic acid core surrounded by a protein coat, with or without an envelope called?

A mature virus particle that contains a nucleic acid core and is surrounded by a protein coat, with the possibility of having an envelope, is referred to as a virion. This term encompasses the complete viral particle that is capable of infecting a host cell. A virion's structure is critical for its function and includes both the genetic material (nucleic acid core) and the protective outer layer (the protein coat or capsid), which helps stabilize the nucleic acids and promote entry into host cells.

In the context of viral classification, the virion represents the fully assembled and infectious stage of the virus, ready to propagate and infect new hosts. The envelope, if present, adds an additional layer of protection and can facilitate the entry of the virus into host cells, but its absence doesn't alter the definition of a virion.

Other terms like capsid denote just the protein coat itself, while nucleocapsid refers specifically to the complex of nucleic acid and capsid proteins, excluding any envelope. Thus, the term virion accurately captures the entire particle in its capable form, making it the correct answer in this context.