Explanation

Write √(17−4√15) = √12 − √5 = 2√3 − √5 (since (√12−√5)² = 17 − 2√60 = 17 − 4√15) and √(8−2√15) = √5 − √3 (since (√5−√3)² = 8 − 2√15). Their sum is √3, option (a).