c. motivation that comes from an outside source, either as an incentive or as the threat of punishment
Extrinsic uses the root ex or exter which means outside. Other words with a similar original are: exit, external, existential. Something that is extrinsic is something that comes from outside of a person. So, extrinsic motivation is when the motivation to do something comes from an outside source.
The opposite of extrinsic motivation is intrinsic motivation. Intrinsic motivation comes from within, a desire to do something or act a certain way for moral reasons.
