Final answer:
A system is a collection of interdependent elements that achieve specific objectives, and according to the given choices, 'a. system' is the correct answer.
Step-by-step explanation:
A system is defined as an object or a collection of objects that work together to achieve specific objectives. According to Essential Knowledge 1.A.1 and 5.A.1, these objects are generally treated as having no internal structure when considered within the context of a system. When we talk about systems within, say, a comparative study of international systems, we're looking at units that act independently but whose behavior is interdependent due to the systemic effects of their collective interaction. Applying this understanding to the student's question, the correct answer is 'a. system' as it involves a set of interdependent elements designed to accomplish specific goals.