**Core Concept**
Calcium absorption is a critical process in maintaining normal serum calcium levels, which is essential for various bodily functions, including muscle contraction, nerve transmission, and bone mineralization. The absorption of calcium occurs mainly in the small intestine, where it is facilitated by specific transport mechanisms.

**Why the Correct Answer is Right**
Calcium absorption is increased by vitamin D, which plays a crucial role in regulating calcium homeostasis. Vitamin D binds to the vitamin D receptor (VDR), which then activates the calcium-sensing receptor (CaSR) in the small intestine, leading to increased calcium absorption. This process is mediated by the parathyroid hormone (PTH)-regulated calcium transport mechanism, which is essential for maintaining normal serum calcium levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not specify any substance or factor that increases calcium absorption.
* **Option B:** This option is incorrect as it does not accurately describe a mechanism or substance that increases calcium absorption.
* **Option C:** This option is incorrect as it does not accurately describe a mechanism or substance that increases calcium absorption.

**Clinical Pearl / High-Yield Fact**
It is essential to note that vitamin D deficiency is a common cause of impaired calcium absorption, leading to osteomalacia in adults and rickets in children. Regular vitamin D supplementation is crucial, especially in individuals with limited sun exposure, malabsorption syndromes, or those with vitamin D-deficient diets.

**Correct Answer: C. Vitamin D**