Which type of flask is not best for measuring volumes accurately?

The beaker is not designed for precise volume measurements. While it can give a rough estimate of liquid volume due to its markings, the graduations on a beaker are not as finely or accurately calibrated as those on a volumetric flask or graduated cylinder. Beakers are generally used for mixing, stirring, and holding liquids rather than for precise measurement.

In contrast, a volumetric flask is specifically designed for accurate measurement of a single volume of liquid, while a graduated cylinder provides more accurate volumetric readings than a beaker but is still not as precise as a volumetric flask. The Erlenmeyer flask can be used for mixing and also features some degree of volume measurement, but like the beaker, it does not provide the same level of accuracy as a volumetric flask. Therefore, among the options listed, the beaker stands out as the vessel least suitable for precise volume measurements, making it the correct choice.