The correct answer is:
The second diagram.
Explanation:
The lines of reflectional symmetry are the lines through which the figure can be folded exactly in half.
A rectangle can be folded in half both vertically through the middle and horizontally through the middle. However, folding a rectangle through its diagram will not fold it exactly in half; one part will overlap the other, they will not meet exactly.
This means the only lines of reflectional symmetry should be horizontally through the center and vertically through the center.