Yoshi is learning that there are many different ways to say "thank you" in Japanese. This use of appropriate conversation demonstrates:

The correct answer is pragmatics, which refers to the rules and conventions for using language effectively in social contexts. In this instance, Yoshi is not just learning the words to express gratitude; he is also understanding the social implications and the situational appropriateness of using various forms of "thank you" in Japanese. This includes recognizing how the context, relationship with the listener, and cultural norms can influence language use, making communication more effective and socially acceptable.

The other options relate to different aspects of language. Syntax deals with the structure and rules governing how sentences are formed. Semantics pertains to the meaning of words and phrases. Morphology focuses on the structure of words and the way they are formed. In this case, Yoshi's learning revolves around the social use of language and its varied expressions, clearly falling under the domain of pragmatics.