How can you tell the difference between subnet addresses and interface addresses?

The network interface address is an network address  that belongs to an interface and defines that interface in the network. Any device with an IP Address, has an IP Address on an interface. The subnet address (or subnet mask) on the other hand defines the routing prefix of an address and divides the IP address into network and host addresses. 
There can be same subnet addresses, but the interface address must be unique defined.

The answer is b; as a lab tech, you’re more behind the scenes and don’t interact with the patients.