**Core Concept**
Vitamin A is a fat-soluble vitamin that plays a crucial role in maintaining vision, immune function, and skin health. Its active forms, including retinol and retinoic acid, influence gene expression and cellular differentiation through specific nuclear receptors.

**Why the Correct Answer is Right**
Retinoic acid (C) is the most potent form of Vitamin A that directly influences DNA by binding to retinoic acid receptors (RARs) and retinoid X receptors (RXRs). These receptors are transcription factors that regulate the expression of target genes involved in cell growth, differentiation, and apoptosis. Retinoic acid's ability to modulate gene expression makes it an essential regulator of embryonic development, tissue homeostasis, and cellular differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinol is an alcohol form of Vitamin A, primarily involved in vision and immune function. While it can be converted to retinoic acid, it does not directly influence DNA.
**Option B:** Retinyl esters are storage forms of Vitamin A, which are hydrolyzed to retinol for utilization. They do not have a direct effect on DNA.
**Option D:** Beta-carotene is a precursor to Vitamin A, but it does not have a direct effect on DNA. It must be converted to retinol and then to retinoic acid to exert its biological effects.

**Clinical Pearl / High-Yield Fact**
Retinoic acid's role in regulating gene expression is essential for maintaining tissue homeostasis and preventing neoplastic transformation. Defects in retinoic acid signaling have been implicated in various cancers, including acute promyelocytic leukemia.

**Correct Answer:** C. Retinoic acid.