The correct option is SCALE.
This is because the experimental data you are interested in is mass and scale is the instrument that is used to measure mass. To use the scale, measure the mass of your water before you put it inside the sun and measure the mass again after you remove it from the sun. The difference in mass is the quantity of water that is evaporated.
There are different type of scale and the one that you use will depend on the size of the material that you want to measure.
