The aspect of language that is most affected when a child misunderstands the meaning of humor in conversation is:

When a child misunderstands the meaning of humor in conversation, the aspect of language most affected is pragmatics. Pragmatics involves the social rules and conventions that guide how language is used in context, including understanding humor, sarcasm, and the nuances of communication. Humor often relies on shared knowledge, cultural references, and the ability to perceive irony or exaggeration, which are all critical components of pragmatic understanding.

In this scenario, if a child struggles with interpreting humor, it suggests they may not fully grasp the contextual cues or the intended social meaning behind the conversation. This difficulty is rooted in their ability to navigate social interactions and understand the subtleties of language use, which is the essence of pragmatics. Other aspects of language, like syntax (the structure of sentences), semantics (meaning of words), and phonology (sounds of language), do not directly address the social understanding needed for humor. Therefore, pragmatics is the right choice, as it directly relates to the social and contextual use of language in conversation.