A. 6

A Tetrahedron has six edges.

OA, OB, OC, AB, AC, BC are the 6 edges.

B. 8

A Rectangular Pyramid has eight edges.

AB, BC, CD, DA, OA, OB, DC, OD are the 8 edges.

C. 12

A Cube has twelve edges.

AB, BC, CD, DA, EF, FG, GH, HE, AE, DH, BF, CG are the edges.

D. 9

A Triangular prism has nine edges.

AB, BC, CA, DE, EF, FD, AD, BE, CF are the9 edges.