Final answer:
The cloud point refers to the temperature at which diesel fuel begins to form paraffin wax crystals. This happens before it reaches the boiling point, which is a separate concept indicating when a liquid transitions into a gas.
Step-by-step explanation:
The temperature at which diesel fuel begins to form paraffin wax crystals is known as the cloud point. Therefore, the correct answer is b) cloud point. The boiling point is the temperature at which a liquid changes its phase to become a gas in a process called evaporation. The boiling point for different substances varies depending on environmental conditions such as pressure. For instance, the boiling point of water is 100 °C at 1.00 atm pressure, but it changes with different pressures. In the context of petroleum refining, a distillation column is used to separate petroleum fractions based on their boiling points; substances with lower boiling points condense at the top of the column while those with higher boiling points condense at the bottom.