Answer : 3. "10th"
Explanation :

The Tenth Schedule was inserted in the Constitution in 1985 by the 52nd Amendment Act. 

It deals with the Anti defection law i.e, provisions as to disqualification on the ground of defection. 

Decision on questions as to disqualification on the ground of detection:- 

If any question arises as to whether a member of a House has become subject to disqualification under this Schedule, the question shall be referred for the decision of the Chairman or, as the case may be, the Speaker of such House and his decision shall be final Provided that where the question which has arisen is as to whether the Chairman or the Speaker of a House has become subject to such disqualification, the question shall be referred for the decision of such member of the House as the House may elect in this behalf and his decision shall be final.