Answer is (C) 90º
Here largest side is 12 cm.
If square of the hypotenuse is equal to square of other two sides, then it is a right angled triangle.
∴ c2 = a2 + b2
AC2 = AB2 + BC2
(12)2 = (63)2 + (6)2
44 = 36 × 3 + 36
144 = 108 + 36
144 = 144
∴ ∆ABC is a right angled triangle and angle opposite to hypotenuse, ∠B = 90°.