The correct answer is a) envelope.
Most viruses have a core structure made up of nucleic acid and proteins, which includes a capsid and capsomeres. The genome refers to the complete set of genetic material within the virus. However, not all viruses have an envelope. The envelope is an outer lipid layer that surrounds some viruses, but many viruses, such as the common cold virus, exist without this additional layer. Therefore, while capsids, nucleic acids, and genomes are present in all viruses, envelopes are not universally found in every viral type.