Correct option is (D) 2 ∠ACB
Angle subtended by an arc at the centre of the circle is double than the angle subtended by that arc at any point on the circumference.
\(\because\) \(\angle AOB\) = Angle subtended by arc AB at centre O
\(\angle ACB\) = Angle subtended by arc AB at point C of circumference
\(\therefore\) \(\angle AOB\) = \(2\,\angle ACB\)