Hey there :)
The highest solubility of a solute will be when it has the greatest quantity in g when dissolved in 100 g of water. So the solute that is most soluble in water, will have a higher g/100 g water.
1:
a: 0°C - Sucrose will be the solute that is most soluble in water.
b: 80 °C - Also Sucrose will be the most soluble in water.
2:
Yes, as we can see in the graph, sucrose, potassium nitrate, ammonium sulfate and sodium chloride have different solubilities between them and at different temperatures. For example, solubility at 0 °C:
sucrose: 180 g / 100 g of water
potassium nitrate: ~10 g/ 100 g of water
ammonium sulfate: 80 g/100 g water
sodium chloride: 30 g/ 100 g of water
