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Objects of class c and D

only C and D. A can't be because it's a C object that means that watever object it is passed it should at least have all the C methods, and atributes, and A doesn't garantied to have.

In this question there is no difference if the classes are polymorphic or not, as no dynamic_cast operator is used.

A* a;
B* b;
C* c;
D* d;

In the following no dynamic_cast is used, so only the last two cases will work
foo (a); // error
foo (b); // error
foo (c); // ok - implicit conversion
foo (d); // ok - type matching

Here occurs implicit conversion, which only converts from derived to base class.

Only C and D even when base classes contain virtual functions. Remember type checking is done at compile time! If A or B is passed, compiler errors will be generated

A can't be - as A is not of type C
B can't be - as B is not related to C at all

1. Objects of class C and D can be passed if the classes are not polymorphic.
2. If Class A is polymorphic the objects of class A,B,C,D can be passed using
dynamic_cast<> where needed.

Depending on the polymorphic nature of the base classes the objects that can be passed will differ.

I think A,C,D classes object can be passed all are polymorphic classes, then by using dynamic_cast it can be done.