Final answer:
The Y chromosome cannot be discerned by light microscopy.
Step-by-step explanation:
The Y chromosome is the chromosome that cannot be discerned by light microscopy. While chromosomes 1, 21, and mitochondrial DNA can be observed by light microscopy, the Y chromosome is much smaller and cannot be visualized without the use of special techniques such as fluorescence in situ hybridization (FISH) or karyotyping.