(b) Five days — ❌ Incorrect.
(a) Three days ✓ — The Book should remain open for a minimum of 3 days during the book building process.
The source passage explicitly states: "The Book should remain open for a minimum of 3 days." The correct answer is (a) Three days. Note: if the price band is revised, the bidding period is extended by 3 more days, subject to a maximum of 10 days total — do not confuse this with the minimum.