Which part of a virus allows it to attach to host cells?

The protein coat of a virus, known as the capsid, plays a crucial role in the virus's ability to attach to host cells. This coat is composed of protein subunits called capsomers, which can form specific structures that interact with receptor sites on the surface of host cells. The interaction between viral proteins and host cell receptors is essential for the virus to enter the host and initiate infection.

The capsid's structure is designed to facilitate this recognition and binding process, often using specific shapes or charges that enable the virus to effectively latch onto the target cell. Once attached, the virus can then proceed to deliver its genetic material into the host, allowing it to replicate and spread.

While the lipid coat is a component of some viruses (particularly enveloped viruses), it is the protein coat that is primarily responsible for the attachment mechanism. The DNA or RNA core contains the viral genetic information but does not play a direct role in the attachment to host cells. Hence, the protein coat is fundamental to the infection process, making it the correct answer.