**Core Concept:** Ogino-Knaus theory is a method of natural family planning (NFP) that uses cervical mucus monitoring to determine if a woman is fertile or infertile. This method is based on the observation of changes in cervical mucus during the menstrual cycle and its relation to ovulation.

**Why the Correct Answer is Right:** Ogino-Knaus theory is based on the Ogino-Knaus criteria, which include the following:

1. **Option C (Correct):** The first day of regular menstrual cycle is defined as day 1, and the first day of a post-menopausal woman is day 1.
2. **Option C (Correct):** The fertile period starts on day 7 (post-ovulation) and lasts until day 14 (pre-ovulation).
3. **Option C (Correct):** The infertile period starts from day 14 (pre-ovulation) and lasts until day 21 (pre-menstruation).
4. **Option C (Correct):** A woman is considered infertile after day 21 if she has a regular menstrual cycle or day 28 if she has an irregular menstrual cycle.

This method involves monitoring the consistency, appearance, and mucus secretion patterns to identify the fertile and infertile periods.

**Why Each Wrong Option is Incorrect:**

**Option A:** This option is incorrect because it does not address the Ogino-Knaus criteria for determining the fertile and infertile periods.

**Option B:** This option is incorrect because it is a sub-criterion of the Ogino-Knaus theory, focusing on the first day of menstruation and the fertile period, but does not encompass the entire theory.

**Option D:** This option is incorrect because it only addresses the appearance of cervical mucus but does not focus on the Ogino-Knaus criteria for determining the fertile and infertile periods.

**Clinical Pearl:** Understanding the Ogino-Knaus theory and applying it correctly is crucial for implementing the natural family planning method effectively. It involves observing cervical mucus changes during the menstrual cycle to predict ovulation, thus determining the fertile and infertile periods. This method can be useful for couples who wish to avoid pregnancy or plan for it, depending on their reproductive goals and knowledge of the Ogino-Knaus criteria.