**Core Concept**
The question tests the understanding of **glycolysis**, a metabolic pathway that converts glucose into pyruvate, releasing energy. The **rate-limiting enzyme** is crucial as it controls the flux through the pathway. In glycolysis, this enzyme is key to regulating the breakdown of glucose.

**Why the Correct Answer is Right**
The correct answer is **Phosphofructokinase-1 (PFK-1)**, an enzyme that catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate. This step is **rate-limiting** because PFK-1 is tightly regulated by various factors, including ATP, citrate, and low pH, which signal the cell's energy status. When energy is abundant, PFK-1 is inhibited, slowing down glycolysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Hexokinase is involved in the initial step of glycolysis but is not typically considered the rate-limiting step in most tissues.
**Option B:** Glucokinase plays a role similar to hexokinase in the liver and pancreas but is not the primary rate-limiting enzyme for the entire glycolytic pathway.
**Option D:** Pyruvate kinase is another important enzyme in glycolysis but is not the primary rate-limiting enzyme.

**Clinical Pearl / High-Yield Fact**
Remember, **PFK-1** is a key regulatory point in glycolysis, and its dysregulation can lead to metabolic disorders. Understanding the regulation of glycolysis is crucial for managing conditions like diabetes.

**Correct Answer:** C. Phosphofructokinase-1.