Which one of the following is not an important use of chlorine in water treatment?

The question you're asking about is question 7: "Which one of the following is not an important use of chlorine in water treatment?"

Let's analyze the options:

• a) Prevention of infection: Chlorine is a disinfectant that kills harmful microorganisms, preventing waterborne infections. This is a primary use.
• b) Prevention of waterborne diseases/re-infection: Chlorine maintains a residual in the water distribution system, preventing pathogens from re-growing and causing re-infection. This is an important use.
• c) Promotes good health: By making water safe to drink, chlorine treatment directly contributes to public health. This is a significant benefit.
• d) Addition of aluminium and magnesium: Chlorine's main function in water treatment is disinfection. It is not used to add minerals like aluminum or magnesium to water. These minerals are either naturally present or added through other processes, not chlorination.

Therefore, the addition of aluminum and magnesium is not an important use of chlorine in water treatment.

The correct option is d).
$\boxed{\text{d) Addition of aluminium and magnesium}}$