Final answer:
The glycoprotein is the component of a virus that extends from the envelope to attach to host cells, enabling the virus to enter and replicate within the host. Hence, option (c) is correct.
Step-by-step explanation:
The component of a virus that is extended from the envelope for attachment to host cells is the glycoprotein. Viruses utilize glycoproteins to latch onto the receptors on the surface of their target host cells. The process of attachment is critical for viral entry and subsequent replication within the host cell. It should be noted that not all viruses have an envelope; however, those with envelopes, like the influenza virus or HIV, have glycoproteins embedded in these envelopes for attachment purpose.
The envelope of a virus is typically derived from the host's membrane structures when the virus buds out of a host cell, incorporating lipids and proteins including its own virus-encoded proteins into its structure. Variations in nucleic acid (some have DNA, others have RNA), the presence of a viral envelope, and the specific use of glycoproteins for attachment exemplify the diversity and complexity of virus structures.