The bread or idli dough rises because of
CGrowth of yeast cells
Yeast is used as leavening agent in bread and idli making. Leavening agent or yeast converts sugars present in dough to carbon dioxide and derive energy from this reaction. Due to this, yeast cells grow and dough also expands. When such dough is baked then air pockets formed due to carbon dioxide will give soft and spongy texture to bread and idli. Therefore, option C is correct.