Answer
$(b)$The magnitude of the component of the field that is perpendicular to the surface
Work Step by Step
The magnetic flux through a surface which is placed in a magnetic field is,
$\phi_B=ABcos\theta$
Here, $\theta$ is the angle between the magnetic field and normal to the surface. So, the term $cos\theta$ gives the component of the magnetic field perpendicular to the surface.
Therefore, the option $(b)$ is correct.
