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b and c
b changes the object size and memory layout of other members. c changes the layout of vtable and how destructor is found.

Here statement d is bit confusing. Let us assume all the scenario.

1) A default value is assigned to the existing argument in the member function
In this case there is no need to recompile the libraries. As the signature of the member function still remains the same.

2) A new argument is added with a default value.
Here recompilation is required for sure. The reason is as mentioned above changes to the member function interface i,e adding a new argument would require to recompile the library.

I think its language specific too. In Java, d requires recompiling dependent libs

should be letter d, because that is sort of changing the interface of the function which it was a bit different in the libraries.

Looks like none actually.
Reasons:
a) Addition of second constructor would be an overloaded one. First constructor(Assuming one already was there still holds good for the client application)
b) New data member addition doesn 't require clt application to rebuild
c)Destructor is called internally when the object is deleted or goes out of scope. So no need to rebuild again
and finally
d)member function with default argument would again would not require clt code change

agree with days

answer is b and C....
and for d if we consider a default value is assigned to the existing argument in the member function..then we don't have to compile..

For d, we need a recompilation. Read more on name mangling. tricky but name mangling uses args of a function for generating names...

what about adding a constructor??...suppose i add a parameterised constructor, will get errors if some base class exists, as base class wil have to provide the values for this new const

In C++, it would be all above.

Basically they all involve changes to the header file while will be included by the libraries use it.

Only exception is if a library only holds pointer or reference to this class but do not call any methods. It can get away with only forward declaration, hence no need to recompile the translation unit.