Concept:
Motivation refers to the internal force, drive, or desire that stimulates a person to act toward achieving a goal.
It may be:
• Positive motivation -- encourages constructive behavior
• Negative motivation -- arises due to fear, punishment, or pressure
Motivation plays an important role in learning, behavior, and personality development.
Step 1: Analyze option A.
Social structure refers to the organized pattern of social relationships and institutions in society.
Hence, option A is incorrect.
Step 2: Analyze option B.
Motivation is the inner force that drives an individual toward a particular action or behavior.
Hence, option B is correct.
Step 3: Analyze option C.
Social defense relates to measures taken for protection of society against harmful activities.
Hence, option C is incorrect.
Step 4: Analyze option D.
Culture refers to socially learned values, customs, and behaviors.
Hence, option D is incorrect.
Therefore, the correct answer is:
\[
\boxed{(B)\ \text{Motivation}}
\]