**Core Concept**
Spermicides are a class of contraceptive agents that work by killing or immobilizing sperm, thereby preventing fertilization. They are commonly found in vaginal foams, creams, and suppositories.

**Why the Correct Answer is Right**
The active agent in these spermicides is indeed nonoxynol-9, a non-ionic surfactant that disrupts the sperm cell membrane, ultimately leading to sperm death. Nonoxynol-9 is the most widely used spermicide and is effective in preventing fertilization when used correctly. Its mechanism of action involves the formation of pores in the sperm cell membrane, causing the release of intracellular contents and ultimately leading to sperm death.

**Why Each Wrong Option is Incorrect**
**Option B:** Levonorgestrel is a progestin used in combination with ethinyl estradiol in oral contraceptives, such as the pill. It is not an active agent in spermicides.
**Option C:** Effectiveness of spermicides is not influenced by the age of the user. However, correct and consistent use of spermicides does increase their effectiveness.
**Option D:** The effectiveness of spermicides is generally lower than that of the diaphragm when used alone. The diaphragm provides a physical barrier against sperm entry, whereas spermicides work by killing or immobilizing sperm.

**Clinical Pearl / High-Yield Fact**
Spermicides are most effective when used in combination with a barrier method, such as the diaphragm or a condom, to prevent fertilization.

**Correct Answer:**
✓ Correct Answer: A. The active agent in these spermicides is nonoxynol-9