Answer
The area used to detect the magnetic field should be lined up so the magnetic field passes through it.
Work Step by Step
To detect the magnetic field of the wave, we need an area for the changing magnetic flux to pass through. As the magnitude of magnetic flux is $A~B~cos~\theta$, ideally we would like the angle between the area and the magnetic field to be zero. Thus, the area used to detect the magnetic field should be lined up so the magnetic field passes through it.
