The correct answer to the question is by increasing their masses, or by decreasing the separation distance between them.

EXPLANATION:

Let us consider two bodies having masses M and M’ respectively . The two bodies are separated from their centre of masses by a distance of R.

As per Newton’s law of gravitation, the two bodies will exert  gravitational force on one another. The gravitational force produced between them is calculated as –

                   Gravitational force F= Gfrac{MM'}{R^2}

Here, G is the gravitational force constant.

From above, we see that the gravitational force is directly proportional to the product of masses of two bodies and inversely proportional to the square of separation distance between them.

Hence, the gravitational force between two objects can be increased by increasing their masses, or by decreasing the distance between them.