1. Perpendicular Bisector of a Triangle

Question

2. Circumcenter
3. Orthocenter
4. Concurrent

Math with the definitions
A. The common point of the perpendicular bisectors of a triangle
B. Segment from a vertex that is perpendicular to the opposite side or line containing the opp. side
C. Segment that is part of a perpendicular bisectors of one of the sides
D. The common point of the angle bisectors of a triangle

Answer 1

these definitions surely can be found in your textbook.
Start with 3A