The correct answer is: B. dust particles
The electrolyte of a lead-acid battery is a mixture of sulfuric acid and water. Dust particles can contaminate the electrolyte and cause problems with the battery. The dust particles can act as a conductor, which can cause a short circuit. They can also block the flow of ions, which can prevent the battery from charging or discharging properly.
Chlorine is not a common impurity in the electrolyte of a lead-acid battery. It is possible for chlorine to be present in the electrolyte if the battery is exposed to chlorine gas. However, this is not a common occurrence.
Lead crystals are not a common impurity in the electrolyte of a lead-acid battery. They can form if the battery is not properly maintained. However, they are not a common problem.
Iron is not a common impurity in the electrolyte of a lead-acid battery. It is possible for iron to be present in the electrolyte if the battery is exposed to iron particles. However, this is not a common occurrence.