**Core concept**: Vocabulary (synonym identification). **Analysis**: 'Radically' means 'in a thorough or fundamental way.' The correct answer is **A. fundamentally**, which directly aligns with this meaning. **Wrong options**: - B. 'slightly' implies minimal change (contradicts the core meaning). - C. 'occasionally' refers to frequency, not degree. - D. 'moderately' suggests partial change, not thoroughness. **Key point**: Synonym identification requires matching the degree and nature of meaning.