The correct answer is A. for coagulation of mud particles.
Alum is a chemical compound with the formula Al2(SO4)3·14H2O. It is a white, odorless, and tasteless powder that is commonly used as a coagulant in water treatment. When alum is added to water, it reacts with the negatively charged particles in the water, such as mud and bacteria, to form larger particles that can be easily filtered out. This process is called coagulation.
B. to remove gases is incorrect because alum does not have the ability to remove gases from water.
C. to kill bacteria is incorrect because alum does not have the ability to kill bacteria.
D. to remove salts is incorrect because alum does not have the ability to remove salts from water.