Answer: the third implementation (option C)

It declares implements PaymentProcessor and gives pay(Decimal amount) a real body, as in the example below:

apex
public class CheckPaymentProcessor implements PaymentProcessor {
public void pay(Decimal amount) {
// functional code here
}
}

PaymentProcessor is an interface, so a class uses it with implements. The class then has to provide a public or global implementation, with a body, of every method the interface declares.

The other images don’t fit:

First: It uses implements, but pay ends in a semicolon with no body. That declares the method without implementing it, so it won’t compile.
Fourth: It uses extends, which is for inheriting from a virtual or abstract class and not for applying an interface. It also has no method body.
Second: The Salesforce logo watermark covers the keyword in the class declaration, so I can’t read it. Since the third image is the clearly valid one, this one most likely has extends, which is wrong for an interface. If it did say implements, it would be identical to the third....

Should be c

My apologies, should actually be C and NOT B. I had a hard time reading which one was interface from the screenshot

Should be B. Uses implements because its an interface and there needs to be functional code as well