In which of the following methods is a small plastic implant placed under the skin?

The method involving a small plastic implant placed under the skin is Implanon. Implanon is a form of long-acting reversible contraception that consists of a small, rod-like implant that is inserted under the skin of a woman's upper arm. This implant releases hormones that prevent ovulation and provide effective birth control for up to three years.

In contrast, an IUD (intrauterine device) is a device inserted into the uterus, not under the skin. Oral contraceptive pills are taken orally and do not involve any surgical placement or implants. A dental dam is a barrier used in dental procedures or safe sex practices, which also does not involve a subcutaneous implant. By understanding how these methods function and where they are administered, it's clear that Implanon is the only option that meets the criteria described in the question.