Which of the following is a non-nucleoside reverse transcriptase inhibitor?

Efavirenz is categorized as a non-nucleoside reverse transcriptase inhibitor (NNRTI) and is primarily used in the treatment of HIV infection. NNRTIs function by binding to the reverse transcriptase enzyme, which is essential for the HIV replication process. By inhibiting this enzyme outside of the active site, efavirenz effectively prevents the conversion of viral RNA into DNA, thereby halting the virus's ability to replicate and spread.

When considering the other options, they belong to different classes of antiretroviral medications. Ritonavir and atazanavir are both protease inhibitors, which work by inhibiting the protease enzyme that HIV uses to cleave newly synthesized polyproteins into functional proteins, which are crucial for the maturation of infectious viral particles. Tenofovir, on the other hand, is a nucleotide reverse transcriptase inhibitor (NRTI) that competes with the natural nucleotides for incorporation into the viral DNA, leading to chain termination.

Thus, the unique mechanism of action of efavirenz as a non-nucleoside reverse transcriptase inhibitor sets it apart from the other options, making it the correct answer.