Identify all the functional interfaces

Four interfaces A, B, C, and D are defined as:

Interface A:

interface A {
  int run(int i);
  int walk(int i);
}

Interface B:

interface B {
  int run(int i);
  String toString();
}

Interface C:

interface C {
 int run(int i);
 default int walk(int i) {
     //..
     return 0;
 }
}

Interface D:

@FunctionalInterface
interface D {
 int run(int i);
}

Which interface(s) of A,B,C, and D is/are Functional Interface(s)? Select Below: