Final answer:
Nuclear pores are the structures that can only be identified using an electron microscope and not with a light microscope due to their small size.
Step-by-step explanation:
The structures in the nucleus that can be identified using an electron microscope but not with a light microscope are the nuclear pores. While structures like the nucleolus, chromosomes, and nuclear envelope can often be discerned with a light microscope, nuclear pores are much smaller and require the higher resolution of an electron microscope to be seen clearly. The nuclear envelope, which encloses the nucleus, contains these protein-lined pores that allow material to move in and out. It is important to note that the diameter of a nuclear pore is approximately 10 nanometers, which is smaller than the resolution limit of a light microscope.