If you're using an up-to-date light microscope to examine animal cells, you won't be able to detect which of the following:

A. bacterial-sized cell bodies
B. the cell nucleus
C. ribosomes
D. cell membranes

From what I've studied, it must be C. ribosomes

But I need a clear answer.

Yup, you got the answer right. Ribosome are too small to be seen with a light microscope.