**Core Concept**
The question tests the student's knowledge of antiviral medications with dual activity against two major viral infections, HIV (Human Immunodeficiency Virus) and HBV (Hepatitis B Virus). This concept involves the pharmacological properties of antiviral agents and their mechanism of action.

**Why the Correct Answer is Right**
The correct answer is a drug that targets both HIV and HBV through different mechanisms. For instance, the drug can inhibit viral replication in HIV by targeting the reverse transcriptase enzyme and in HBV by inhibiting the viral DNA polymerase. This dual activity makes the drug effective in managing co-infection of HIV and HBV in patients.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not have dual activity against both HIV and HBV. It may be an antiretroviral drug used to treat HIV but lacks efficacy against HBV.
**Option B:** This option is incorrect because it is not commonly used to treat both HIV and HBV. It may be a nucleoside analog, but its primary use is not for dual antiviral activity.
**Option C:** This option is incorrect because it is primarily used to treat HBV and not HIV. It may be a nucleotide analog, but it lacks activity against HIV.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that patients co-infected with HIV and HBV require antiviral medications with dual activity to effectively manage both infections. This is crucial for preventing disease progression and reducing the risk of liver-related complications.

**Correct Answer:** C. Tenofovir disoproxil fumarate (TDF)