If the environment surrounding a cell has a lower concentration of solute than the cell:

If the environment surrounding a cell has a lower concentration of solute than the cell is in the hypotonic environment.  

If concentrations of solutes are less outside the cell than inside, that means that there is more water outside. When a cell is exposed to such hypotonic conditions, there is a water movement into the cell via osmosis. The cell is going to swell under this condition.

It would be most likely be prophase because of the chromosomes duplicating. but without the picture, it’s really hard to tell