Which one of the following is an emotion?

The correct answer is Option Fear.

In psychology, an emotion is a complex state of feeling that results in physical and psychological changes that influence thought and behavior. It is typically a subjective experience accompanied by physiological responses and behavioral or expressive reactions.

Let's analyze the given options:

1. Fear: Fear is a primary, fundamental emotion triggered by a perceived threat or danger. It prepares an individual to react through responses like fight or flight.

2. Memory: Memory refers to the mental capacity or cognitive process of encoding, storing, and retrieving information. It is a cognitive function, not an emotion.

3. Attention: Attention is the cognitive process of selectively focusing on a discrete aspect of information, whether subjective or objective, while ignoring other perceivable information. It is a cognitive control mechanism.

4. Stimulus: A stimulus is an external or internal event, agent, or change in the environment that evokes a specific functional reaction in a cell, tissue, or organism. It is a cause or trigger of a response, not an emotion itself.

Therefore, among the given options, Fear is the only emotion.