Answer:
The correct answer is - option a.
Explanation:
The eustachian tubes joins the pharynx and middle ear and this is the one of the way the infections of the upper respiratory infections affect the middle ear too.
As per the mother's statement it is clear that it develop due to the upper respiratory infections because of opening of the Eustachian tubes not by entic reasons as it occur immediate after the upper respiratory infections.
Thus, the correct answer is - option A.