B.The water molecules in the black can had the largest increase in average kinetic energy.
Step-by-step explanation:
Here, black painted can absorbs more heat than the other color painted cans.
Black color absorbs all the heat and didn't reflect anything back, so it absorbs the most heat.
White color reflects all the heat, so heat absorbed by the white can is least.
When the black can absorbs heat then the water molecules in the can gets its maximum amount of kinetic energy so that the water molecules in the can collide with each other and also along with the walls of the can here, and so the average kinetic energy increases.