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 isosceles trapezium.

When we reflect or fold it in the line a, we get:

When we reflect it in the line b, we get:

This is a MATCH!

When we reflect it in the line c, we get:

When we reflect it in the line d, we get:

It is only in line b that we get a match.

So line b is the only line of symmetry.