Final answer:
Physics is the branch of STEM that primarily tries to understand how things work, from the basic principles of matter and energy to the interactions that govern the universe's behavior.
Step-by-step explanation:
The branch of STEM that primarily attempts to find out how things work is Physics. Physics is fundamentally concerned with uncovering the principles that govern the universe, from the smallest particles to the largest structures. It aims to explain the composition, structure, and changes in matter, as well as how various aspects of the universe interact. This includes understanding energies involved in processes, such as in chemical reactions where physics can determine the amount of energy required or released. Physics also plays a crucial role in developing new technologies and processes in engineering disciplines. Therefore, the correct answer is C. Physics.