what happens when RBC is kept in 0.91% solution of NaCl?

The concentration of salt inside the RBC is about 0.9%. When the RBC is placed in a solution of salt having exactly the same concentration, the 2 solutions (the salt solution and cell cytoplasm) are isotonic, and hence there is no osmosis occurring.

When the concentration of the salt solution outside is greater, water flows from the cell to the outer solution (by osmosis) and hence the cell shrinks.

When the concentration of the salt solution outside is lesser, water flows into the cell from the outer solution (by osmosis) and hence the cell swells up and might burst.