Read the scenario.

To find the sample volume, we need to subtract the initial water level of 20.00 mL from the final water level of 28.50 mL.

So, the sample volume = final water level - initial water level

Sample volume = 28.50 mL - 20.00 mL

Sample volume = 8.50 mL

The answer to the question is 8.50 mL.

Since the initial water level is measured to 2 significant figures (20.00 mL), we should report the answer to the same number of significant figures.

Therefore, the correct answer is 8.50 mL.