Question

Why is the induced emf maximum when armature is parallel to the magnetic field?

Answer 1

The Electromotive force force or in other words, the induced voltage is proportional to the rate of change of the magnetic field. Hence it is proportional to the change in the overall magnetic flux that crosses the moving armature. the plane of the coil/armature. Now, when the armature/coil is perpendicular to the magnetic field, small change in its angle leads to only small absolute change in magnetic flux. Similarly to how tiny angle change near 90 degrees does not change the sine of that angle much, when compared to the same change of the angle near 0 degrees, the change in sine value of it is substantial.
Hence the reason is geometric, based on the rotation of the armature/magnetic field.

hope this helps you out. .....