Final answer:
The first step you should take is to call the agency and report your findings.
Step-by-step explanation:
If you arrive at a client's home and they don't answer the door, the first step you should take is to call the agency and report your findings (C). This is important because they need to be aware of the situation and can provide guidance on how to proceed. The agency may instruct you to try gaining access through a window (D) or they may decide to contact the client's family (B) or emergency medical services (A) depending on the circumstances.