Which operator is used to access data members of the outer structure?

The operator used to access data members of the outer structure is the dot operator (.) in many programming languages like C, C++, Java, etc.

Here is a step by step guide on how to use it:

1. Define your structure. For example, in C++ you might have a structure like this:

struct Outer {
    int x;
    struct Inner {
        int y;
    };
};

2. Declare a variable of your structure type:

Outer outer;

3. You can access the data member 'x' of the outer structure directly using the dot operator:

outer.x = 10;

4. If you have a nested structure and you want to access the data member of the inner structure, you would use the dot operator twice:

Outer::Inner inner;
outer.inner.y = 20;

In this case, the first dot is accessing the 'inner' structure inside 'outer', and the second dot is accessing the 'y' data member inside 'inner'.