Concept:The phrase “subjected to” is a fixed expression used to say someone is made to experience something unpleasant.Explanation:The sentence describes the robbers receiving “a thorough beating” from the mob.The correct preposition after “subjected” is “to”.“Subjected to” means forced to undergo or experience something.The other options do not form the correct idiomatic expression.Answer:The robbers were subjected to a thorough beating by the irate mob.Correct option: A. to