Answer: The correct answer is temperature.
Step-by-step explanation:
The solubility is defined as the amount of solute that can be dissolved in a given amount of solvent at specific temperature.
We are given that the solubility of NaCl is 35.9g per 100g at 20°C.
If we dissolve 35.9 grams in 100 grams of water, it is completely dissolve at this temperature.
If the temperature increases, the solubility also increases and if temperature decreases, the solubility decreases.
It is given that some solid remains undissolved, so the temperature would have been decreased, therefore the solubility decreases.
Hence, the correct answer is temperature.