Common structure across animal and plant cells

Steps:

• Identify key features: human red blood cells lack nucleus but contain cytoplasm; leaf palisade cells have chloroplasts and cell walls; human liver cells have nuclei.
• Compare structures: cytoplasm is present in all three cell types as the fluid matrix.
• Eliminate unique features: cell walls and chloroplasts are plant-specific; nuclei are absent in mature red blood cells.
• Conclude X labels the universal component.

Why C is correct:

• Cytoplasm is the semifluid substance filling cells, essential for organelles and reactions in all eukaryotic cells per cell theory.

Why the others are wrong:

• A: Cell walls provide rigidity in plants but are absent in animal cells like red blood and liver cells.
• B: Chloroplasts enable photosynthesis in plant cells like palisade but are missing in human cells.
• D: Nuclei control cell functions in most cells but mature red blood cells eject theirs for oxygen transport.

Final answer: C