**Core Concept**
The question is testing the knowledge of safe and effective contraceptive methods for lactating mothers. Lactational amenorrhea, the natural postpartum infertility period, typically lasts for 6 months postpartum. However, the mother may need protection before this period ends. The correct answer should be a method that is safe and effective for breastfeeding women.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends that breastfeeding women use safe and effective contraceptive methods to space their pregnancies. Among the options, the most appropriate method is the **progestin-only pill (POP)**, also known as the mini-pill. This is because it is safe for breastfeeding women and does not affect milk production or the baby's health. The POP works by preventing ovulation, thus preventing pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the combined oral contraceptive pill (COCP) contains estrogen, which can decrease milk production and affect the baby's health.

**Option B:**
This option is incorrect because the intrauterine device (IUD) can cause infection in breastfeeding women, especially if it is inserted too soon after delivery.

**Option C:**
This option is incorrect because the diaphragm is not an effective method of contraception during the first 6 months postpartum, and it may also cause irritation to the vaginal mucosa.

**Clinical Pearl / High-Yield Fact**
The progestin-only pill (POP) is a safe and effective method of contraception for breastfeeding women because it does not affect milk production or the baby's health.

**Correct Answer:** C. Progestin-only pill.