**Core Concept**
The American Cancer Society recommends regular mammography for women of a certain age group to detect breast cancer early, thereby improving survival rates. This is based on the understanding that the risk of breast cancer increases with age, and mammography is an effective screening tool for women over 40.

**Why the Correct Answer is Right**
The correct age for regular mammography is 40 years due to the increased incidence of breast cancer in this age group. Breast cancer risk doubles every 10 years after the age of 40, making mammography an essential tool for early detection. The American Cancer Society and other reputable organizations recommend annual mammography screening for women aged 40-74 years. This is because mammography can detect breast cancer at an early stage, when it is more treatable and curable.

**Why Each Wrong Option is Incorrect**
**Option A:** 20 years is too early for regular mammography, as the risk of breast cancer is relatively low in this age group. Most breast cancers are diagnosed in women over 50 years old.
**Option B:** 30 years is also too early for regular mammography, as the risk of breast cancer is still relatively low, and mammography may not be as effective in younger women due to denser breast tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to note that women with a family history of breast cancer, genetic predisposition, or other risk factors may need to start mammography screening earlier, typically at 10-year intervals, starting at age 25-30.

**✓ Correct Answer: C. 40**