When an object of one class is a data field within another class, they are related by
a.
composition
b.
extension
c.
is-a
d.
scope

a. composition

Explanation:

When an object of one class is a data field in another class, the two classes are related by composition. In particular the containing class is composed using the contained class. For example:

class container {

    contained  c;

    int a;

}

class contained {

    int b;

}

Here the container class is containing an instance of contained class creating a composition relation.