Which line is a line of symmetry of ΔABC?A) line b B) line a C) line c D) line d

Line of symmetry cuts the intended figure/graph/line etc. into 2, where the 2 parts reflect each other exactly about the line of symmetry itself.

Line a is the answer, since line a cuts triangle ABC into 2 triangles, which reflect each other exactly about line a. Triangle ABdot and triangle CBdot are congruent and reflect each other about line a. ("dot" refers to the point that has the right-angle sign in the figure)

Hope this helps! :)