Final answer:
TLR 3 is the toll-like receptor that is specifically activated by viruses, recognizing double-stranded RNA to trigger an antiviral immune response.
Step-by-step explanation:
The toll-like receptor that is specifically activated by viruses is TLR 3. Toll-like receptors (TLRs) are a class of proteins that play a crucial role in the innate immune system, recognizing molecular patterns associated with microbial pathogens. TLR 3 recognizes double-stranded RNA, a molecule often found in viruses, triggering an immune response to viral infections.