**Core Concept**
Selective Estrogen Receptor Modulators (SERMs) interact with estrogen receptors to produce tissue-specific agonist or antagonist effects. In the context of GnRH agonist therapy, SERMs are used to mitigate estrogen deprivation effects on bone density and lipid profiles.

**Why the Correct Answer is Right**
SERMs act as estrogen receptor agonists in bone and lipid tissues, thereby maintaining bone density and preventing lipid profile disturbances associated with estrogen deprivation caused by GnRH agonist therapy. This is particularly important in the treatment of hormone-sensitive cancers, such as breast and prostate cancer. SERMs selectively modulate estrogen receptor activity, allowing for the preservation of beneficial effects on bone and lipid metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because SERMs are not typically used to stimulate tumor growth.
**Option B:** This option is incorrect because GnRH antagonists do not require add-back therapy with SERMs.
**Option C:** This option is incorrect because aromatase inhibitors are not used concurrently with GnRH agonists and SERMs.

**Clinical Pearl / High-Yield Fact**
SERMs are a crucial component of add-back therapy in the management of hormone-sensitive cancers, allowing for the preservation of bone density and lipid profiles while minimizing the risk of estrogen deprivation effects.

**Correct Answer:** C.