Which part of the alimentary canal is most acidic?

The stomach is a muscular sac-like organ situated in the upper abdomen, between the oesophagus and the small intestine. The stomach is the most acidic part of the alimentary canal. 

It secretes a strong acid i.e., hydrochloric acid (HCl), which aids in the breakdown of food and kills harmful bacteria or other microorganisms that may be present. 

The acidic environment promotes the activation of digestive enzymes such as pepsin, which aid in the breakdown of proteins in food.

Hence, the correct answer is stomach.