It assumes a sign function, i.e try replacing x and y, the equation which classifies the dataset properly is the right answer.

e.g for case four.

-1 -1 -.5 = -2.5 since it turned out negative call it Class C_{1}

-1+1 -.5=-.5 class C_{1}

1-1-.5=-.5 class C_{1}

and finally

1+1-.5-=1.5 since this is positive call it C2

Note: option D is the only equation that classifies them properly hence the answer.