The onion cells are plant cells, and plant cells have cell membrane and cell wall both. The cell wall is composed of the cellulose, and it provides rigidity to the cell and helps the cell to maintain its shape. That is why when an onion cell is put in distilled water, it did not burst.
However, the red blood cells do not have cell wall. They only have cell membrane, which cannot provide rigidity to the cell, and therefore, it bursts when put in distilled water.
Hence, the correct answer is option (1) - 'the red blood cells only have cell membrane which does not protect them from bursting'.