Assertion (A) is correct: Middle ear infections are indeed more common in children due to the anatomical differences in their ear structure.
Reason (R) is also correct: Children have a shorter, more horizontal, and wider Eustachian tube compared to adults. This makes it easier for infections to travel from the throat to the middle ear, increasing the likelihood of ear infections.
Since Reason (R) explains the cause behind the increased frequency of middle ear infections in children, both (A) and (R) are correct and (R) is the correct explanation of (A).