Riya drank 1/4 of the 500 mL milk bottle. To find out how much is left, we need to subtract the amount she drank from the total amount.
The amount she drank can be calculated as 1/4 * 500 mL = 125 mL.
So, the amount left would be 500 mL - 125 mL = 375 mL.
Therefore, there are 375 mL of milk left in the bottle.