**Core Concept**
Hyaluronic acid is a type of glycosaminoglycan that plays a crucial role in maintaining the structure and hydration of connective tissue. It is a key component of the extracellular matrix, particularly in the skin, eyes, and joints.

**Why the Correct Answer is Right**
Hyaluronic acid is present in the vitreous humor of the eye, where it maintains the gel-like consistency and provides structural support. It also helps to regulate the amount of water in the eye and prevents the vitreous humor from becoming too dense. In the skin, hyaluronic acid is involved in the process of wound healing and tissue repair. It is also present in the synovial fluid of joints, where it helps to reduce friction and facilitate smooth movement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyaluronic acid is not primarily associated with the nervous system.
**Option B:** This option is incorrect because hyaluronic acid is not primarily associated with the cardiovascular system.
**Option C:** This option is incorrect because hyaluronic acid is not primarily associated with the respiratory system.

**Clinical Pearl / High-Yield Fact**
Hyaluronic acid is often used in cosmetic procedures, such as dermal fillers, to restore volume and hydration to the skin.

**Correct Answer:** D. Synovial fluid, vitreous humor, and skin.