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