Gravitational attraction between two objects depends on their masses and the distance between them. In this case, two identical solid copper spheres are considered.

1. Gravitational Attraction
The gravitational attraction between two objects can be given by Newton's Law of Gravitation:

F = G * (m1 * m2) / r^2

Where:
F is the gravitational force between the two objects,
G is the gravitational constant,
m1 and m2 are the masses of the two objects, and
r is the distance between the centers of the two objects.

2. Identical Solid Copper Spheres
Since the two copper spheres are identical, their masses are the same. Let's assume the mass of each sphere is M.

3. Distance Between the Spheres
When the two spheres are placed in contact with each other, the distance between their centers is equal to twice the radius of each sphere, which is 2R.

4. Calculation of Gravitational Attraction
Substituting the values into the equation, we have:

F = G * (M * M) / (2R)^2
F = G * M^2 / 4R^2

5. Proportional Relationship
To determine the proportional relationship, we can ignore the gravitational constant G, as it is a constant value.

F ∝ M^2 / R^2

From this, we can see that the gravitational attraction is inversely proportional to the square of the distance (R^2) between the spheres.

6. Answer
The question asks for the relationship with respect to the radius R. As the gravitational attraction is inversely proportional to R^2, it means it is directly proportional to 1/R^2.

Therefore, the correct answer is option 'C', which states that the gravitational attraction is proportional to R^4.