Which is the least likely to be used by astronomers to investigate galaxies?

Astronomers are least likely to use optical telescopes for investigating galaxies that are very distant or obscured by interstellar dust, as these telescopes primarily detect visible light. Instead, they often rely on radio, infrared, or X-ray telescopes, which can penetrate dust and reveal more information about such galaxies. Optical observations are still valuable, but their limitations make them less effective for certain types of galaxy studies.