Answer
We can rank the situations according to the amplitude of the net wave on the string:
$c \gt a \gt b$
Work Step by Step
When the angle between the phasors is zero, the two waves are in phase, and there is fully constructive interference. The net wave would have the maximum possible amplitude.
When the angle between the phasors is $180^{\circ}$, then there is fully destructive interference. The net wave would have an amplitude of zero.
Situation $c$ is closest to an angle of zero, so the net wave has the largest amplitude.
Situation $b$ is closest to an angle of $180^{\circ}$, so the net wave has the smallest amplitude.
We can rank the situations according to the amplitude of the net wave on the string:
$c \gt a \gt b$
