**Core Concept**
Nonoxynol-9 is a nonionic surfactant used as a spermicidal agent in the prevention of pregnancy. It works by disrupting the membrane of sperm cells, ultimately leading to their death and preventing fertilization.

**Why the Correct Answer is Right**
Nonoxynol-9 is a type of spermicide that acts by denaturing proteins on the surface of sperm cells, causing them to lose their motility and viability. This process is facilitated by the surfactant properties of nonoxynol-9, which allows it to interact with and disrupt the cell membranes of sperm cells. The effectiveness of nonoxynol-9 as a spermicide is largely due to its ability to penetrate the cell membrane of sperm cells and cause the release of potassium ions, leading to cell death.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable (no information provided).
* **Option B:** Not applicable (no information provided).
* **Option C:** Not applicable (no information provided).
* **Option D:** Not applicable (no information provided).

**Clinical Pearl / High-Yield Fact**
It's essential to note that spermicides, including nonoxynol-9, should be used in conjunction with other forms of contraception, such as condoms, to provide optimal protection against pregnancy. Additionally, spermicides should not be used in conjunction with other vaginal products, as this may reduce their effectiveness.

**Correct Answer: D. Spermicide. Nonoxynol-9 is a spermicide marketed as 'Today'.**