Final answer:
Glycogen is a substrate for glycogenolysis and is directly converted to monosaccharides, which enter the glycolytic pathway.
Step-by-step explanation:
Glycogen is a substrate for glycogenolysis, which directly converts it into monosaccharide units. The process of glycogenolysis breaks down glycogen to glucose-1-phosphate (G1P), which is then converted to glucose-6-phosphate (G6P) and can enter glycolysis.
This allows ATP to be produced for a longer time during exercise or when blood sugar levels drop. Therefore, the correct answer to the question is a. Glycolysis.