**Core Concept**
Centchroman, also known as ormeloxifene, is a non-steroidal selective estrogen receptor modulator (SERM) used as a contraceptive. It works by inhibiting estrogen receptors in the endometrium and pituitary gland, thereby reducing the growth of the endometrium and ovulation.

**Why the Correct Answer is Right**
Centchroman is administered orally in a dose of 30 mg per day for 12 days in the first cycle, followed by 30 mg every 12 days for 6 months to maintain effectiveness. The mechanism of action involves the inhibition of estrogen receptors in the endometrium, leading to a decrease in endometrial proliferation and ovulation. This results in the prevention of implantation of the fertilized egg.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because the dose of centchroman is not 25 mg.
**Option B:** This option is likely incorrect because the dose of centchroman is not 40 mg.
**Option C:** This option is likely incorrect because the dose of centchroman is not 20 mg.

**Clinical Pearl / High-Yield Fact**
Centchroman is an effective contraceptive option for women who cannot use hormonal contraceptives due to medical reasons. It is also used to treat dysmenorrhea and menorrhagia.

**Correct Answer:** C. 30 mg