The interjection 'Hi' expresses
a. Surprise
b. Call for attention
c. Shock
d. Relief

The correct answer is Option D (b only).

Key Points:
• Concept: Functions of Interjections in English Grammar. An interjection is a word used to express emotion or to attract attention.
• Explanation: The interjection 'Hi' (or 'Hey') is conventionally used as an informal greeting or to call for attention from someone. Statement 'b' accurately identifies this function.
• Other Statements: Interjections for surprise include 'Oh!', shock 'Gosh!', and relief 'Phew!'.
• Official Key: Option 4 ('b only') is the correct answer according to the official key.
• Key Takeaway: Remember: 'Hi' serves as a call for attention or a standard greeting interjection.