**Core Concept**
The **calendar method**, also known as the **rhythm method**, is a form of natural family planning that involves tracking a woman's menstrual cycle to predict fertile and infertile days. This method is based on the understanding of the **menstrual cycle** and its relation to **ovulation**.

**Why the Correct Answer is Right**
Although the options are not provided, the calendar method is often attributed to **Knaus** and **Ogino**, who independently developed this method in the 1930s. They observed that the menstrual cycle is divided into phases of **follicular development** and **luteal phase**, allowing for the prediction of fertile windows.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's challenging to provide a detailed explanation. However, any option that does not correspond to Knaus or Ogino would be incorrect.
**Option B:** Similarly, without the option, it's difficult to provide a reason.
**Option C:** and **Option D:** would also be incorrect if they do not match the known developers of the calendar method.

**Clinical Pearl / High-Yield Fact**
The calendar method has a relatively high failure rate compared to other contraceptive methods, primarily because it assumes a regular 28-day menstrual cycle, which is not the case for many women.

**Correct Answer:** D. Knaus-Ogino.