Option 1 states that both the assertion (A) and the reason (R) are true, and that the reason is the correct explanation of the assertion.

The assertion states that red blood cells burst when placed in water. This is true because red blood cells contain a higher concentration of solutes (such as ions) inside them compared to the concentration of solutes in pure water. As a result, water molecules move into the red blood cells in an attempt to equalize the concentration of solutes on both sides of the cell membrane, through a process called osmosis. When water enters the red blood cells, they swell and eventually burst.

The reason given in option 1 states that this bursting of red blood cells is due to osmosis, which is also correct. Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In the case of red blood cells, the higher concentration of solutes inside the cells causes water to enter the cells, leading to their bursting.

Therefore, option 1 correctly states that both the assertion and the reason are true, and that the reason provides the correct explanation for the assertion.