To find:
What a pendulum is?
Step-by-step explanation:
A system is a matter defined by the walls of space.
A surrounding is an environment around the system in observation.
A pendulum is a system containing the bob of the pendulum and the string to which the bob is attached.
The rest of the environment around the pendulum is considered as the surrounding.
Final answer:
The pendulum the student has chosen to study is a system.
Thus the correct answer is option a.