Which of the following objects has the greatest density
line a
line b
line c
all are the same density
not enough information

The density is determined on the steepness of the slope. The greater the density is bases upon the steepest slope. To conclude, I'd say Line A has the steepest slope therefore has the greatest density.

Density is measured in mass per unit of volume, so in this case g/mL. We can see that the three objects have the same volume, but different masses. Line A has the greatest mass, so will have the greatest volume as it has more mass per mL, meaning that there's more of the stuff in one container.