Final answer:
The population under Chapter 143 of the Government Code is determined by the decennial census conducted by the U.S. Census Bureau, influencing representation in Congress and the distribution of federal resources.
Step-by-step explanation:
The population under subsection (a)(1) of Chapter 143 of the Government Code is determined by the census that is conducted every ten years. The United States Census Bureau is responsible for conducting this census, which is a detailed enumeration of the population. This census is critical because it determines the apportionment of representatives in Congress from each state, and how federal resources are distributed. Legislation, such as Article I, Section 2 of the U.S. Constitution, supports the systematic count of the population, ensuring that representation is fairly allocated based on the most recent census data.