Which of the following structures is best observed using a compound light microscope?

• A. DNA double helix
• B. Viral capsid
• C. Ribosomes
• D. Bacterial cell wall

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the resolution limits of a compound light microscope. Typically, it can resolve structures down to about 200 nanometers (0.2 micrometers) in size.
2. Step 2: Consider the size of each structure: DNA double helix (~2 nanometers in diameter), viral capsid (20-300 nanometers), ribosomes (~20-30 nanometers), and bacterial cell wall (thickness around 20-80 nanometers but part of a whole bacterial cell which is about 1-10 micrometers).
3. Step 3: Recognize that the compound light microscope cannot resolve very small structures like DNA double helix, viral capsids, or ribosomes individually because they are below the resolution limit.
4. Step 4: Note that bacterial cells, including their cell walls, are large enough to be seen under a compound light microscope, making the bacterial cell wall the best observed structure among the options.
5. Step 5: Conclude that the bacterial cell wall is the correct answer because it is part of a structure visible at the scale resolvable by a compound light microscope.