**Core Concept**
Postmenopausal women with estrogen receptor (ER) positive breast cancer require endocrine therapy to block estrogen's stimulatory effect on cancer cell growth. This involves inhibiting estrogen production or its action on cancer cells.

**Why the Correct Answer is Right**
The most appropriate drug for postmenopausal women with ER positive breast cancer is aromatase inhibitors (AIs). Aromatase inhibitors work by blocking the enzyme aromatase, which is responsible for converting androgens to estrogens in peripheral tissues. By inhibiting aromatase, AIs decrease estrogen production, thereby reducing the growth stimulus for ER positive breast cancer cells. Examples of aromatase inhibitors include anastrozole, letrozole, and exemestane.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not specified, so we'll assume it's a placeholder. However, if it were a specific drug, it would be incorrect if it's not an aromatase inhibitor or is not the most appropriate choice for postmenopausal women with ER positive breast cancer.
**Option B:** This option is also not specified, but if it's a non-steroidal anti-inflammatory drug (NSAID) or a different class of drug, it would be incorrect because it's not the most appropriate choice for ER positive breast cancer.
**Option C:** This option is not specified, but if it's a chemotherapy agent, it would be incorrect because the question is asking about endocrine therapy, which is a different treatment approach for ER positive breast cancer.

**Clinical Pearl / High-Yield Fact**
Aromatase inhibitors are generally considered more effective than tamoxifen in postmenopausal women with ER positive breast cancer, especially in terms of disease-free survival.

**Correct Answer: A. Anastrozole.**