title says it all...
Im confused i don't get why...
The is wrong because when it says ALL, it means that EVERY symmetric line is a diagonal.
This can be proved wrong with a counterexample.
This here is a symmetric line, but not a diagonal
A diagonal is just a line from a vertex to any other vertex.
Try it with a normal peice of rectangular paper.
Fold it along the diagonal.
Does it fold exactly in half and do the 2 sides overlap each other perfectly?
If not, then that diagonal is NOT an axis of symmetry.