A cell will swell up if________________.

• The cell swells up when endosmosis occurs. Endosmosis is a process in which the water molecules move from outside to the inside of the cell through the cell membrane. 
• This occurs when the solution is hypotonic, i.e., the cell has a lower solute concentration than the surrounding medium. In another words when the concentration of water molecules in surrounding medium is higher than water molecules concentration in the cell.

So, the correct answer is option B.