Final answer:
To determine the effects of collisions between galaxies, astronomers mainly use computer simulations, as they allow detailed modeling of the complex gravitational interactions between galaxies over long time scales.
Step-by-step explanation:
Astronomers utilize a variety of tools to study the effects of galaxy collisions. One of the primary tools used for this research is computer simulations. These simulations allow astronomers to model the complex interactions between stars, gas, and dust over the course of hundreds of millions of years. By doing so, they can observe the potential outcomes of such collisions, including the merging of galaxies into a single entity if the collision is sufficiently slow.
While space telescopes are crucial for observing galaxies and acquiring data, it is through the use of sophisticated computer simulations that the long-term evolution of colliding galaxies can be studied. Therefore, the correct answer to the question "Which tools do astronomers use to determine the effects of collisions between galaxies?" would be d) computer simulations.