**Core Concept**
Selective estrogen receptor modulators (SERMs) are a class of compounds that act on the estrogen receptor (ER) to selectively modulate estrogen's effects in various tissues. They can mimic or block estrogen's actions depending on the tissue type, making them useful in treating conditions such as breast cancer, osteoporosis, and menopausal symptoms.

**Why the Correct Answer is Right**
SERMs work by binding to the estrogen receptor, either activating or inhibiting its activity in different tissues. For example, tamoxifen (A) and raloxifene (C) are SERMs that primarily activate estrogen receptors in bone and fat tissue, leading to increased bone density and reduced fat mass, while inhibiting estrogen receptors in breast tissue, reducing the risk of breast cancer. Toremifene (D) also has SERM activity, although it is less potent than tamoxifen. On the other hand, letrozole (B) is an aromatase inhibitor, which reduces estrogen production in the body by inhibiting the aromatase enzyme.

**Why Each Wrong Option is Incorrect**
**Option B:** Letrozole is an aromatase inhibitor, not a SERM, as it reduces estrogen production rather than modulating estrogen receptor activity.
**Option D:** Toremifene is a SERM, although it is less potent than tamoxifen.

**Clinical Pearl / High-Yield Fact**
SERMs are a useful class of compounds for managing estrogen-related conditions, but their effects can vary depending on the specific tissue type and disease state.

**Correct Answer: B. Letrozole. Letrozole.**