To determine which classes are declared properly, we need to consider the inheritance hierarchy and ensure that there are no conflicts or circular dependencies. The base class is A, while B and C are derived from A. Option A (class Class_3(A,C): pass) is incorrect because class C is already a child of class A, leading to a conflict. Option B (class Class_2(B,C): pass) is correct because B and C are derived from A, and there is no conflict or circular dependency. Option C (class Class_4(A,B): pass) is incorrect because class B is already a child of class A, leading to a conflict. Option D (class Class_1(C,B): pass) is correct because both C and B derive from A without any conflicts. Thus, the properly declared classes are found in options B and D.