Q:

What UML relation to use for class A and class C, where class C is a member of Class B and Class A depends on Class B

Here is a code example. 
class C;

/* B is a container of class C */
class B
{
public:
    size_t size() const;

    C const& operator[](size_t index) const {
        return elements[index];
    }

    std::vector <C> elements;
}

class D
{
public:
    D(C const& c);
}

/* A is a container of class D */
class A
{
public:
    A(B const& b) {

        // TO construct Class D instance, A needs to pass reference to class C 
        // to class D's constructor.
        for (int i = 0; i < b.size(); ++i) {
            elements.emplace_back(b[i]);
        }
    }
    std::vector <D> elements;
}

Here,

A COMPOSITION D
B COMPOSITION C
A DEPENDENCY B
D DEPENDENCY C

Which UML relation to use between A and C ? 
It is unidirectional Association from A to C ? 

A:

This is what you  are saying (not so sure about your code since my Cxx knowledge is limited):

As you can clearly see, A depends on C as well since it's a member of B. But you don't need to express that with a dependency since it's obvious.
Note: According to the comment of @Ister A does not directly depend on C since it does not use it. The UML diagram shows that. The dependency is only indirect since (likely) B needs C to work propertly. Which is why C was put inside B first hand. But drawing a dependency from A to C would be wrong here.

