**Core Concept**
Nonoxynol 9 is a nonionic surfactant spermicidal agent used in contraceptive foams, jellies, and suppositories. It disrupts the cell membrane of sperm, ultimately leading to their death.

**Why the Correct Answer is Right**
Nonoxynol 9 works by interacting with the sperm membrane, causing its disruption and eventual lysis. This is due to its surfactant properties, which allow it to bind to and solubilize the cholesterol and phospholipids in the sperm membrane. This leads to the release of enzymes from the sperm, resulting in its death. The mechanism of action of nonoxynol 9 is not dependent on the presence of cervical mucus or the female reproductive tract environment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the mechanism of action of nonoxynol 9.
**Option B:** This option is incorrect because it implies that nonoxynol 9 works through a hormonal mechanism, which is not the case.
**Option C:** This option is incorrect because it suggests that nonoxynol 9 is a barrier method of contraception, which is not accurate.

**Clinical Pearl / High-Yield Fact**
It's essential to note that nonoxynol 9 has been associated with an increased risk of vaginal irritation and condom breakage due to its detergent properties. Therefore, its use should be limited to specific situations and with proper precautions.

**Correct Answer: D. Nonoxynol 9 is a nonionic surfactant spermicidal agent.**