The correct option is A Both the assertion and the reason are true and the reason is a correct explanation of the assertion.
Leavening may be achieved by the process of fermentation, which releases carbon dioxide gas. Bakers yeast, composed of living cells of the yeast strain Saccharomyces cerevisiae, is available as a pressed cake and in a powdered form. When added to doughs, yeast initiates fermentation by acting upon certain sugars contributed by other dough ingredients, releasing both carbon dioxide and substances that affect the flavour and aroma of the baked product.