**Core Concept**
Emergency contraceptives, also known as postcoital contraceptives, are hormonal medications or devices used to prevent pregnancy after unprotected sex. They work by inhibiting or delaying ovulation, fertilization, or implantation of a fertilized egg.

**Why the Correct Answer is Right**
The effectiveness of emergency contraceptives depends on the timing of administration after unprotected intercourse. **Option C** states that emergency contraceptives are effective if administered within 120 hours (5 days) after unprotected intercourse. This is because most emergency contraceptives, such as levonorgestrel (Plan B) and ulipristal acetate (ella), are most effective when taken within 72-120 hours of unprotected sex. After this period, the window of effectiveness decreases significantly. The mechanism behind emergency contraceptives involves the inhibition of ovulation, fertilization, or implantation through the use of hormonal agents such as prostaglandin analogs, gonadotropin-releasing hormone (GnRH) antagonists, or selective progesterone receptor modulators.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the timing of emergency contraceptive administration is critical for effectiveness. Taking emergency contraceptives too soon after unprotected intercourse may not be necessary, and waiting too long may render them ineffective.
**Option B:** This option is incorrect because the timing of emergency contraceptive administration is not a straightforward 24-48 hours after unprotected intercourse. The effectiveness of emergency contraceptives decreases significantly after 120 hours.

**Clinical Pearl / High-Yield Fact**
It is essential to note that emergency contraceptives are not a substitute for regular contraception and should not be used as a primary method of birth control. Additionally, emergency contraceptives may not be effective in preventing pregnancy if taken after implantation of a fertilized egg has occurred.

**Correct Answer:** C. 120 hours (5 days)