Final answer:
To increase the gravitational force of attraction between two objects, you can either increase the mass of one object or decrease the distance between the objects.
Step-by-step explanation:
The gravitational force of attraction between two objects can be increased by increasing the mass of one object (option a) or by decreasing the distance between the objects (option d).
According to Newton's law of universal gravitation, the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the distance between them.
For example, if you increase the mass of one object, while keeping the distance between the objects constant, the gravitational force of attraction between them will increase. Similarly, if you decrease the distance between the objects while keeping their masses constant, the gravitational force will also increase.