**Core Concept**
Chromosomes are complex structures composed of DNA and proteins that carry genetic information. To visualize chromosomes, light microscopy is used, but its resolution is limited, preventing the direct observation of individual chromosomes.

**Why the Correct Answer is Right**
The resolution of a light microscope is approximately 200-250 nanometers. This is due to the wavelength of visible light used in light microscopy. As a result, chromosomes, which are much smaller than this resolution limit, cannot be directly visualized. To overcome this limitation, techniques like Giemsa staining or banding are used to enhance contrast and distinguish between different regions of the chromosome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete and does not provide any information.
**Option B:** This option is incorrect, as it does not provide a valid resolution limit.
**Option C:** This option is incorrect, as it does not match the known resolution limit of a light microscope.

**Clinical Pearl / High-Yield Fact**
To visualize chromosomes, techniques like karyotyping, fluorescence in situ hybridization (FISH), or chromosome painting are used, which provide higher resolution and allow for the identification of specific genetic abnormalities.

**Correct Answer: D. 200-250 nanometers**