**Core Concept**
Substrate-level phosphorylation is a type of phosphorylation reaction in which a high-energy phosphate group is transferred directly from a high-energy compound to ADP, generating ATP. This process is crucial in glycolysis, where it helps to drive the conversion of glucose into pyruvate.

**Why the Correct Answer is Right**
The correct answer is seen in the step where phosphoglycerate kinase (PGK) catalyzes the conversion of 1,3-bisphosphoglycerate (1,3-BPG) to 3-phosphoglycerate (3-PG), generating one ATP molecule in the process. This reaction is an example of substrate-level phosphorylation, where the high-energy phosphate group from 1,3-BPG is directly transferred to ADP, producing ATP. The enzyme PGK uses the energy from the phosphate bond in 1,3-BPG to drive this reaction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a step in glycolysis where substrate-level phosphorylation occurs.
* **Option B:** This option is incorrect because it refers to the conversion of phosphoenolpyruvate (PEP) to pyruvate, which is catalyzed by pyruvate kinase, not a step where substrate-level phosphorylation occurs.
* **Option D:** This option is incorrect because it refers to the conversion of glucose to fructose-1,6-bisphosphate, which is catalyzed by phosphofructokinase-1, not a step where substrate-level phosphorylation occurs.

**Clinical Pearl / High-Yield Fact**
Substrate-level phosphorylation is a critical mechanism in glycolysis that helps to generate ATP from the breakdown of glucose. This process is essential for the proper functioning of the cell, particularly in situations where oxidative phosphorylation is impaired.

**Correct Answer:** C.