**Core Concept**
The protective factor for ovarian cancer refers to a biological or lifestyle factor that reduces the risk of developing ovarian cancer. This concept is crucial in understanding the prevention and early detection of ovarian cancer.

**Why the Correct Answer is Right**
The correct answer is **Parity**, specifically, having given birth to one or more children. Studies have consistently shown that women who have had children have a lower risk of developing ovarian cancer compared to those who have never been pregnant. This protective effect is thought to be due to the hormonal changes that occur during pregnancy, which may alter the growth and development of ovarian tissue. The exact mechanism is not fully understood, but it is believed to involve changes in the levels of estrogen and progesterone, as well as the growth of new ovarian tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a recognized protective factor for ovarian cancer. While certain dietary factors may have a protective effect, there is no conclusive evidence to support this option.

**Option B:** This option is incorrect because it is not a well-established protective factor for ovarian cancer. While certain lifestyle factors may influence cancer risk, this option is not supported by strong scientific evidence.

**Option C:** This option is incorrect because it is actually a risk factor for ovarian cancer. Women who have never been pregnant or have a history of infertility may be at increased risk of developing ovarian cancer.

**Clinical Pearl / High-Yield Fact**
Women who have had children are at lower risk of developing ovarian cancer, and this protective effect may be related to hormonal changes during pregnancy.

**Correct Answer: D. Parity**