To convert milliliters to liters, you can use the conversion ratio where 1 liter equals 1,000 milliliters.
Given that the cylinder holds 6,750 milliliters, you can convert it to liters as follows:
\[ \text{Liters} = \frac{\text{Milliliters}}{1,000} = \frac{6,750}{1,000} = 6.75 \text{ liters} \]
Therefore, the correct answer is 6.75.