A spectrophotometer measures the transmittance or the absorbance, or both, of a particular wavelength of light after it has passed through a liquid sample. Before the transmittance or absorbance of the sample is measured, a cuvette filled only with solvent, called the blank, is placed in the spectrophotometer and measured. Select the reason that, after the blank is measured, the cuvette must be placed in the spectrophotometer in the same orientation each time that it is used.
a. The spectrophotometer will break if the cuvette position is changed during the experiment.
b. The transmittance of the cuvette must be measured in the same place each time.
c. The cuvette will only fit into the spectrophotometer in one orientation.
d. The transmittance of the liquid must be measured in the same place each time.

When using a spectrophotometer, light passes not only through the liquid sample, it also passes through the cuvette. This means that each time a reading is made, you not only measure the transmittance/absorbance of the sample, but of the cuvette as well.

For this reason it's important that the reading of the cuvette's absorbance remains the same through all the process, so the answer is b), because different faces of the cuvette may have different absorbances.