When we talk of a line of symmetry, we mean a line that passes through a plane shape and divides the shape into exactly two equal halves.
Any line that passes through a shape and gives us two other shapes which are complements of each other such that when they are joined together gives back the original shape.
In the question, we can identify three lines , l , m and n and we want to select which of these lines are lines of symmetry.
How do we go about this?
We simply look at both sides of the line, if what we have on both lines are equal, then what we have is a line of symmetry.
Looking at each line drawn , we can see that only line m divides the shape in a way that the two pieces can be overlapped.
Although the other two lines cut the shape into two, they do not give that overlapping image that line m would give
Hence, for this shape, line m is the line of symmetry
