**Core Concept**
The intrauterine contraceptive device (IUCD) is a type of reversible contraceptive that is inserted into the uterus to prevent pregnancy. It works by causing a foreign body reaction in the uterus, leading to an inflammatory response that prevents implantation of the fertilized egg.

**Why the Correct Answer is Right**
The correct answer is Copper. Copper IUCDs work by releasing ions that are toxic to sperm and cause an inflammatory response in the uterus, making it difficult for a fertilized egg to implant. The copper ions also induce a change in the cervical mucus, making it more difficult for sperm to penetrate. The copper IUCD is a popular choice for long-term contraception due to its high efficacy and ease of use.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the type of material used in the IUCD. While some IUCDs may be made of other materials, copper is the most common and well-studied material used in IUCDs.
**Option B:** This option is incorrect as it does not accurately describe the composition of IUCDs. While some IUCDs may be made of plastic or other materials, copper is the primary material used in IUCDs.
**Option C:** This option is incorrect as it is not a common material used in IUCDs. While some IUCDs may be made of other materials, copper is the most widely used and effective material.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the type of IUCD used may affect the risk of complications, such as pelvic inflammatory disease (PID). Copper IUCDs are generally considered safe and effective, but non-copper IUCDs, such as those made of plastic or hormones, may have different risks and benefits.

**Correct Answer:** C. Copper