To determine the reservation percentage for the Economically Weaker Section (EWS) under the Indian Constitution:
1. The Indian Constitution provides a $10 _%$ reservation for the Economically Weaker Section.
2. This reservation is intended to provide opportunities to those who are economically disadvantaged but not covered under other reservation categories.
3. The other options do not reflect the correct percentage as specified in the Constitution.
Therefore, the correct answer is (3) $10 _%$.