Understand that glycolysis is the metabolic pathway that breaks down glucose to extract energy for cellular metabolism.
Recall that glycolysis starts with a 6-carbon sugar molecule, which is the substrate for the pathway.
Identify the molecule options given: Glyceraldehyde 3-phosphate (3 carbons), Fructose 1,6-bisphosphate (6 carbons but is an intermediate), Pyruvate (3 carbons, end product).
Recognize that the initial molecule entering glycolysis is glucose, a 6-carbon sugar, before it is converted into intermediates like fructose 1,6-bisphosphate.
Conclude that glycolysis begins with glucose, the 6-carbon molecule that is phosphorylated and processed through the pathway.