Ontario Grade 11 University Biology Practice Exam 2025 – Your All-in-One Guide to Exam Success!

The correct choice is "Capsid" because the capsid is the protein shell that encases and protects the nucleic acids (either DNA or RNA) of a virus. This structure is crucial for the virus as it serves not only as a protective barrier against environmental factors but also plays a role in the recognition and attachment to host cells, aiding in the viral infection process.

The capsid is composed of protein subunits called capsomers, which come together to form the protective shell. This structural organization is vital for maintaining the integrity of the viral genetic material until it is delivered into a host cell.

Other structures, such as the capsule, cell wall, and envelope, pertain to different biological contexts. The capsule generally refers to a protective layer found around some bacterial cells, the cell wall is a rigid layer that provides structural support to plant and bacterial cells, and the envelope is an additional lipid layer surrounding some viruses, which can assist in their infectivity but does not serve the same fundamental protective and structural roles as the capsid does.