Psychology defines motivation as a state of the organism that arouses activity, keeps it going and directs it towards a goal. The words drive, need and urge that appear in every standard definition all point to a condition lying within the learner.

Rewards, praise, marks and punishment given from outside can certainly start an activity, but they do so only when they awaken some want within the learner. By themselves they are incentives offered to the person, while the stimulating condition that actually sets behaviour in motion remains inside.

Describing the stimulant merely as unseen says nothing about where it arises and would equally fit many outer forces that simply go unnoticed, so that description does not answer what is asked. Since a precise statement of the stimulating condition is available among the alternatives, the denial choice cannot be accepted either.

Internal option is correct.