The instant the conductors are moving parallel to the magnetic field, they do not cut any lines of flux. A indicates zero. At 90 they are cutting through a maximum number of lines of flux and at maximum angle. At the same time the white conductor cuts UP through the field. The induced emfs in the conductors are series-adding.
B reads maximum value.
They no longer cut through any lines of flux. The meter again reads zero. From 0 to 180 the conductors of the armature loop have been moving in the same direction through the magnetic field. Therefore, the polarity of the induced voltage has remained the same. This is shown by points A through C on the graph.