**Core Concept**: Vocabulary (synonym differentiation). **Analysis**: The key distinction lies in the intensity and duration of the emotion. 'Startle' specifically refers to a sudden, brief shock (e.g., a loud noise causing a jump). 'Horrify' (A) and 'terrify' (B) imply prolonged/extreme fear, while 'frighten' (D) is a general term without emphasizing suddenness. **Error Check**: A/B/D lack the 'suddenness' implied in the question.