b. stem c. bulb
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An onion is considered a bulb, which is a type of stem that grows underground. At the base of the bulb is a basal plate from which roots and the layers, or scales, of an onion grow. The scales store starch and nutrients when environmental conditions are not good for growth. These nutrients are released and help the plant grow quickly when environmental conditions become favorable again. The dry and crunchy thin outside scales of the onion serve a protective function.