The comparison between a boy's and a girl's fifteenth birthday can vary depending on cultural context. However, in many cultures, particularly in Latin American traditions, a girl's fifteenth birthday, known as a "quinceañera," is a significant event celebrated with a large party and often includes religious ceremonies and traditions. In contrast, a boy's fifteenth birthday typically doesn't have the same level of cultural significance and celebration.
Given this context, the most accurate answer would be:
D. the boy's fifteenth birthday party is not as big an event as the girl's.