**Core Concept**
The mechanism of Intrauterine Contraceptive Device (IUCD) involves altering the uterine environment to prevent implantation of a fertilized ovum. IUCDs release progestins, which create a hostile environment for implantation by thickening cervical mucus, thinning the endometrium, and altering the uterine cavity.

**Why the Correct Answer is Right**
The primary mechanism of IUCDs involves the release of progestins, which induce endometrial atrophy. This atrophy reduces the endometrial thickness, making it unfavorable for implantation. Additionally, progestins cause cervical mucus to thicken, preventing sperm penetration. The progestin released by the IUCD also causes the uterine cavity to become more acidic, creating an unfavorable environment for implantation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the mechanism of IUCDs is primarily progestin-mediated, and non-progestin IUCDs (copper IUCDs) work by inducing a local inflammatory response, which prevents implantation.

**Correct Answer:** A.