**Core Concept**
The **Human Papillomavirus (HPV)** vaccine, specifically Gardasil, is designed to protect against certain strains of HPV that are associated with cervical cancer and other diseases. Gardasil is a quadrivalent vaccine, meaning it protects against four different HPV types.

**Why the Correct Answer is Right**
Gardasil protects against HPV types 6, 11, 16, and 18. HPV types 16 and 18 are high-risk strains that are responsible for about 70% of cervical cancer cases. HPV types 6 and 11 are low-risk strains that are commonly associated with genital warts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Gardasil does protect against HPV type 6.
**Option B:** This option is incorrect because Gardasil does protect against HPV type 11.
**Option C:** This option is incorrect because Gardasil does protect against HPV type 16.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Gardasil protects against the most common high-risk and low-risk HPV types, but it does not protect against all HPV types, such as type 31 and 45.

**Correct Answer:** D. 31