Jason's mother has five children. The first one is July. The second one is August. The third one is September. The fourth one is October. What is the fifth child's name?
The fifth child is
Jason's four siblings have been described. That means that the fifth one, the one remaining, is Jason.
The initials of July, August, September, October, and November spell Jason. Therefore the seemingly-right answer of November, which we reject because we're told she's Jason's mother, can in fact be right if "Jason" is the family nickname for all the children the way "the girls" or "the rugrats" is in some families.
Since the question states that Jason's mother has five children, the fifth must be named Jason.