**Core Concept**
The Depo Medroxyprogesterone Acetate (DMPA) injection is a long-acting reversible contraceptive method that works by suppressing ovulation and altering the cervical mucus to prevent sperm penetration. DMPA is a progestin-only contraceptive that is administered subcutaneously every 3 months to maintain its contraceptive efficacy.

**Why the Correct Answer is Right**
DMPA's contraceptive efficacy is dose-dependent and is maintained as long as the injection is administered every 3 months. This frequency is crucial to prevent the loss of contraceptive effectiveness, which can occur if the injection is administered too infrequently or too frequently. The 3-monthly interval allows for the gradual decline of medroxyprogesterone levels to a level that supports ovulation and fertility if the injection is not administered on schedule.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering DMPA monthly would result in excessively high levels of medroxyprogesterone, leading to potential side effects such as irregular bleeding, weight gain, and mood changes.

**Option C:** Administering DMPA every 6 months would lead to a decrease in contraceptive efficacy, as the peak levels of medroxyprogesterone would not be maintained, allowing for ovulation and fertility.

**Option D:** Administering DMPA yearly would result in a significant loss of contraceptive efficacy, as the levels of medroxyprogesterone would not be sufficient to suppress ovulation and alter the cervical mucus.

**Clinical Pearl / High-Yield Fact**
It's essential to note that DMPA's contraceptive efficacy is not affected by the timing of the menstrual cycle, making it a convenient option for women with irregular cycles or those who are breastfeeding. However, it's crucial to inform patients that DMPA may delay the return of fertility after discontinuation.

**✓ Correct Answer: B. 3 Monthly. DMPA is administered subcutaneously every 3 months to maintain its contraceptive efficacy.**