▲15-year-old这类词有两个用法：

一是用作形容词，用于名词前作定语。如：

The 46-year-old author has finally hit the big time. 这位 46 岁的作家终于走红了。

Six-year-old Gemma had been left alone in the house. 六岁的杰玛被单独留在了房子里。

It’s not very dignified behaviour for a 54-year-old man. 对于一个54岁的人来说，这种行为太有失体面了。

The court heard how the mother had beaten the 11-year-old boy. 法庭听取了这个母亲如何暴打那个11岁男孩的经过。

二是直接作用名词，表示多大岁数的人（为可数名词，与其他普通名词一样，可以连用不定冠词或用复数）。如：

The test was administered to all 11-year-olds. 这项测试的实施对象为所有 11 岁的孩子。

He was not considered competent to teach seven-year-olds. 他被认为没有能力教7岁的孩子。

Her command of language is very advanced for a six-year-old. 对于一个6 岁的孩子来说，她对语言的掌握已是很超前了。

Even a six-year-old can have an accident at night sometimes. 即使是六岁大的孩子，晚上有时也可能会尿床。

▲现在回到你的问题来：你的句子中的 fifteen-year-olds 就是 fifteen-year-old 用作名词时的复数形式；泛指15岁的孩子，应该用复数形式。当然，若选项 C 加上不定冠词也是对的，即：

No way! I don't think a fifteen-year-old should be allowed to drive.（正确）