**Core Concept**
Bone resorption is a process by which osteoclasts break down and degrade bone tissue, releasing minerals such as calcium into the bloodstream. This process is crucial for bone remodeling and maintaining calcium homeostasis. The regulation of bone resorption involves various hormonal and local factors.

**Why the Correct Answer is Right**
The correct answer involves identifying factors that decrease bone resorption. Estrogen, for instance, has a protective effect on bone by inhibiting osteoclast activity and thereby reducing bone resorption. Other factors such as calcitonin, a hormone produced by the thyroid gland, also decrease bone resorption by inhibiting osteoclast activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucocorticoids, such as cortisol, actually increase bone resorption by promoting osteoclast activity and suppressing osteoblast activity.

**Option B:** This option is incorrect because parathyroid hormone (PTH) increases bone resorption by stimulating osteoclast activity and promoting the release of calcium from bone.

**Option C:** This option is incorrect because vitamin D, while crucial for calcium absorption, also increases bone resorption by promoting the activity of osteoclasts.

**Clinical Pearl / High-Yield Fact**
Estrogen deficiency, such as that seen in postmenopausal women, leads to increased bone resorption and a higher risk of osteoporosis.

**Correct Answer:** D. Estrogen.