**Correct option is (B) 90°**

\(\because\) AM = BM = 4 cm (given)

It implies M is mod-point of chord AB.

But M is intersection point of OM and AB.

It implies OM bisect chord AB.

Since, O is centre of the circle.

Therefore, OM is perpendicular to chord AB.

\((\because\) Any line passing through centre of the circle & bisect the chord is perpendicular to the chord)

\(\Rightarrow\) OM \(\bot\) AB

\(\therefore\) \(\angle OMB=\angle OMA\) \(=90^\circ\)