Final answer:
To increase the electric force between two charged particles, you should increase the charge of at least one particle or decrease the distance between them, as per Coulomb's law.
Step-by-step explanation:
The electric force between two charged particles can be increased by either increasing the charge of one or both particles or by decreasing the distance between them. According to Coulomb's law, the electric force between two charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. Hence, to increase the electric force: Increase the charge of one of the particles (option c). Decrease the distance between the particles (option d). In the context of Coulomb's law, options a and b where the charge is decreased would result in a decrease in the electric force.