Concept: This question tests your understanding of the functions of the limbic system and its close association with the hypothalamus in regulating various physiological and behavioral responses.

Why (D) is correct: The limbic system, often called the 'emotional brain', is a complex set of brain structures located on top of the brainstem and buried under the cortex. It is intimately connected with the hypothalamus. Together, they play crucial roles in:
A) Regulation of sexual behaviour: The limbic system, particularly the amygdala and hippocampus, along with the hypothalamus, influences sexual drive and behavior.
B) Expression of emotional reaction (e.g., excitement, pleasure, rage and fear): This is a primary function of the limbic system, with the amygdala being central to processing emotions like fear and aggression, and the hypothalamus mediating the physiological responses to these emotions.
C) Motivation: The limbic system, especially its reward pathways, is deeply involved in motivation, learning, and memory formation related to emotional experiences.

Why other options are wrong: Since all the listed functions (sexual behaviour, emotional reactions, and motivation) are indeed regulated by the limbic system in conjunction with the hypothalamus, options A, B, and C are individually correct but incomplete. Option D encompasses all these correct functions.

NEET Tip: Remember that the limbic system is not a single structure but a collection of interconnected brain regions (e.g., amygdala, hippocampus, cingulate gyrus) that work together with the hypothalamus to control emotions, memory, and motivation.

Correct Answer: (D)