**Core Concept**
The question requires knowledge of contraceptive methods suitable for commercial sex workers, who are at a higher risk of sexually transmitted infections (STIs) and unintended pregnancies. Effective contraception should also offer protection against STIs.

**Why the Correct Answer is Right**
The correct answer is **Implantable Contraceptive Device (IUD)**, specifically the Levonorgestrel-Releasing Intrauterine System (LNG-IUS). This method is highly effective in preventing pregnancy and also provides protection against STIs, including HIV. The LNG-IUS releases a progestin hormone that thickens the cervical mucus, making it difficult for sperm to penetrate, and also has a local effect on the endometrium, reducing the risk of STIs.

**Why Each Wrong Option is Incorrect**
* **Option A:** Barrier methods like condoms are important for STI prevention but may not be as effective as IUDs in preventing pregnancy.
* **Option B:** Oral contraceptives may offer some protection against STIs but are not as effective as IUDs and may have side effects.
* **Option C:** Emergency contraception like Plan B is not a reliable long-term solution for commercial sex workers and may not prevent STIs.

**Clinical Pearl / High-Yield Fact**
Commercial sex workers should use a dual-method approach, combining a highly effective contraceptive method like an IUD with a barrier method like condoms to prevent both unintended pregnancies and STIs.

**Correct Answer: D. Implantable Contraceptive Device (IUD).**