Final answer:
The number of representatives in the House of Representatives is determined by the population of each state as per the U.S. Constitution. The correct option is B.
Step-by-step explanation:
The part of the legislative branch where the number of representatives is determined by population is the House of Representatives.
According to Article 1, Section 2 of the U.S. Constitution, representatives are apportioned among the states based on their respective populations, as determined by the census which is conducted every ten years.
The Senate, on the other hand, consists of an equal number of senators from each state, which is two per state, regardless of the state's population. The correct option is B.