A line of symmetry divides a shape into two matching parts, which will overlap each other exactly when folded along the line of symmetry.
Look at this regular hexagon.
When we reflect or fold it in the line a, we get:
This is a MATCH!
When we reflect it in the line b, we get:
This is not a MATCH!
When we reflect it in the line c, we get:
This is a MATCH!
When we reflect it in the line d, we get:
This is a MATCH!
So lines a, c and d are lines of symmetry.
Of course, a regular hexagon has a total of 6 lines of symmetry, but not all of them were shown.