To solve this problem, we need to determine the net gain of ATP when one molecule of glucose is converted to two molecules of pyruvic acid through glycolysis.$\newline$Let's break down the glycolysis process:$\newline \bullet$Glycolysis is the metabolic pathway that converts glucose (C$_6$H$_ {12}$O$_6$) into two molecules of pyruvic acid (C$_3$H$_4$O$_3$).$\newline \bullet$During glycolysis, a total of 4 ATP molecules are produced through substrate-level phosphorylation.$\newline \bullet$However, 2 ATP molecules are consumed in the initial steps of glycolysis to phosphorylate glucose and fructose-6-phosphate.$\newline \bullet$Therefore, the net gain of ATP from glycolysis is calculated as follows:$\newline$Net ATP gain$= 4$ (produced) - 2 (consumed) = 2 ATP.$\newline$Thus, the net gain of ATP when one molecule of glucose is converted to two molecules of pyruvic acid is 2.$\newline$This corresponds to Option 3.