The correct answer is: C. iron
Water used for paper mills should not contain iron because it can cause the paper to become discolored and brittle. Iron can also react with other chemicals in the papermaking process to form compounds that can be harmful to the environment.
Magnesium, sodium, and chlorine are all common minerals that are found in water. They are not harmful to paper and can actually be beneficial in some cases. For example, magnesium can help to improve the strength of paper, while sodium can help to prevent the paper from becoming too acidic. Chlorine is often used to disinfect water, which can be helpful in preventing the spread of disease.
However, iron is a different story. Iron can cause a number of problems for paper mills. It can react with other chemicals in the papermaking process to form compounds that can be harmful to the environment. It can also cause the paper to become discolored and brittle. This can make the paper less attractive and less durable.
For these reasons, it is important to remove iron from water before it is used in a paper mill. This can be done by using a variety of methods, such as filtration, ion exchange, or precipitation.