a) Yeast are the single cell organisms which perform anaerobic respiration. The dough rose because of the release of carbon dioxide which is a product of anaerobic respiration.
b) The dough becomes sour because of the production of alcohol produced during the anaerobic respiration by Yeast.
c) A pinch of sugar is added to the dough for yeast to feed upon it and multiply.
d) If the dough is kept in refrigerator as soon as it is prepared there would be no change in it. Yeast requires warm climate to grow and multiply and as there is low temperature inside a refrigerator, there would not be the perfect climate for yeast to grow.