If the Indifference Curves for Some Individual Are Vertical Straight

If the indifference curves for some individual are vertical straight lines, the utility function is:

A) U(x,y) = x + 2y.
B) U(x,y) = xy.
C) U(x,y) = x² + 5.
D) U(x,y) = 2xy^{2 .}