The macromolecule must be starch.
Explanation:
Starch is a polymer of Glucose joined by glycosydic bond.
Starch is the main energy storage substance in green plants.
Starches react with iodine and give a bluish black or black color and thus iodine test is considered as a confirmatory test for starches.
In this experiment ,the solution turns very dark black on addition of iodine so we can be sure that the solution contained starch.
