Answer: Option 'a' is correct.
Explanation:
Since we have given that ∠NSM,
we need to find is vertical angles of ∠NSM,
1) ∠OSL is a vertical angle of ∠NSM as it has one common vertex and all other are collinear points.
2) ∠SMN can't be vertical angle as vertical angles must be equal at common vertex but it does not do that it forms angle on M rather than on S.
3) ∠SNM can't be vertical angles as vertical angles must be equal at common vertex but it does not do that it forms angle on N rather than on S.
4) ∠OSN can't be vertical angle as vertical angles must have only one common vertex but in this case it has two common vertex i.e. S and N. so it will be linear pair.
Hence, Option 'a' is correct.