Step 1: Splenic cysts are divided into two broad groups, true (primary) cysts that have an epithelial or parasitic lining, and false (secondary) cysts that lack a lining.
Step 2: Among true cysts, the parasitic group is by far the largest. Worldwide the most common splenic cyst overall is the hydatid (echinococcal) cyst, caused by Echinococcus granulosus. In endemic regions it accounts for the majority of all splenic cysts.
Step 3: A pseudocyst (option C) is the most common type of false cyst and usually follows trauma. In Western, non-endemic populations pseudocysts may be the commonest seen locally, but for the worldwide most common splenic cyst the hydatid cyst is the standard answer expected in this exam.
Step 4: Dermoid cysts (option B) and lymphangiomas (option D) are rare congenital true cysts and are not the most common.
Step 5: Therefore the most common cyst of the spleen is the hydatid cyst.
Answer: A. Hydatid cyst.