Final answer:
Darwin's Theory of Evolution opened the intellectual door for psychologists to study instinct as a potential grand theory of emotion.
Step-by-step explanation:
The scientific event that opened the intellectual door for psychologists to study instinct as a potential grand theory of emotion was b. Darwin's Theory of Evolution. Darwin's theory proposed that humans and animals share a common ancestry and that behaviors and emotions are rooted in biological adaptations for survival and reproduction. This concept provided a framework for psychologists to study instinctual behaviors and emotions as a product of natural selection.