Final answer:
The Güira is the instrument that sets the fundamental tempo in the rhythm of merengue music, making it the correct answer to the student's question.
Step-by-step explanation:
The instrument that regulates the fundamental tempo in the rhythm of merengue music is the Güira. The Güira is a metal scraper from the Dominican Republic that produces a distinctive rasping sound which sets the pace of the rhythm in merengue. Unlike the Kuba drums, which are tied to royalty and ceremonies, the Güira's function is primarily rhythmic and foundational in this particular genre of music. Therefore, the correct answer to the student's question is "C) Güira".