**Core Concept:** Emergency contraception (EC) refers to the use of pharmacological agents for preventing pregnancy after unprotected sexual intercourse or contraceptive failure. The most effective form involves the administration of a highly hormonal intervention within 12-72 hours after unprotected sexual intercourse. The most common EC methods include levonorgestrel-containing pills and copper intrauterine devices (IUDs).

**Why the Correct Answer is Right:** Option C, "oral contraceptive pills", is not used as emergency contraception due to the time window. Oral contraceptive pills require a regular regimen for contraception and are least effective when taken after unprotected intercourse. They are less effective when started late and are not recommended for emergency contraception.

**Why Each Wrong Option is Incorrect:**

A. "Intrauterine device (IUD):" IUDs are highly effective contraceptives when placed correctly and used appropriately. However, they are not suitable for emergency contraception due to the time window. IUDs must be inserted within 5 days of unprotected intercourse for maximum efficacy.

B. "Intravenous immunoglobulin (IVIG):" IVIG is a medication used to treat immune deficiencies and autoimmune disorders. It is unrelated to emergency contraception and should not be used as a contraceptive method.

D. "Intravenous magnesium sulfate:" Magnesium sulfate is primarily used in the treatment of eclampsia, pre-eclampsia, and status epilepticus. It is not used for emergency contraception due to its lack of efficacy and contraindications in this context.

**Clinical Pearl:** Emergency contraception should be administered as soon as possible after unprotected intercourse to maximize its effectiveness. In practice, copper IUDs and levonorgestrel-releasing intrauterine systems (IUS) are the most commonly used emergency contraceptives. In case of missed pills, or the use of barrier methods, or failure of contraception due to other reasons, these medications can prevent pregnancy for up to 72 hours post-coitus.

**Correct Answer:** C. Intrauterine device (IUD)

The IUD is highly effective in preventing pregnancy when inserted within five days of unprotected intercourse. However, due to the time window for efficacy, it is not considered a suitable emergency contraceptive option.