A sphere is released on a smooth inclined plane from the top. When it moves down its angular momentum is

Question

$\begin{array}{1 1} a) \text{conserved about every point} \\ \text{b) conserved about the point of contact only} \\ \text{c) conserved about the centre of sphere only} \\ \text{d) conserved about any point on a fixed line parallel to the ibclined plane and passing through the centre of the ball.} \end{array}$