The correct answer is option 4. A and B only.

Alum (potassium aluminium sulfate) is a chemical compound commonly used as a coagulant in water purification. Coagulation is a process in which small, suspended particles are grouped together to form larger particles that settle out of the water. This process is effective in removing mud particles from water.

Sedimentation is the process of settling out of suspended particles from water. While alum does aid in coagulation, which leads to sedimentation, it is not the primary mechanism by which alum purifies water.

Dialysis is a process of separating molecules based on their size and charge. Alum is not used in the dialysis process.

Forming a true solution with water is not a process involved in water purification. True solutions are homogeneous mixtures in which the solute is evenly distributed throughout the solvent. Alum, when added to water, dissociates into its constituent ions, forming an electrolyte solution.

Therefore, the only correct answer is A and B only. Alum helps in purifying water by coagulating and settling out mud particles.