Anew multicellular organism has been discovered, and scientists want to study the organism’s cell structures, including organelles. which microscope should they use?

The answer is transmission electron microscope.

A compound light microscope is not useful in observing structures inside the cells because they have not as high resolution as a scanning electron microscope and a transmission electron microscope. The latter two give much higher resolution, but the scanning electron microscope allows the study of the surface of the objects. So, it would not be possible to look at the internal structures of the cell on the scanning electron microscope. On the other hand, the transmission electron microscope allows studying a very thin cross-section of the cell. It is best for looking at internal structures of the cell, including organelles.