When Mr. Rizk, who is 89 years old, goes to the post office, the staff there often speak to him slowly and in a high-pitched tone. Mr. Rizk has no hearing problems and is mentally competent and he often finds this behavior insulting. Mr. Rizk is being subjected to:

elderspeak

Explanation:

Based on the scenario being described within the question it can be said that Mr. Rizk is being subjected to elderspeak. This term refers to a unique speech style in which younger adults speak to older adults using simpler vocabulary and sentence structure, and many times in a louder-than-normal volume. Such as the individuals at the post office are doing to Mr. Rizk thinking that Mr. Rizk would only understand them if they speak that way.