When measuring the volume expansion of a liquid, the liquid must be held in a container. When the liquid is heated, the container also gets heated and expands. The observed increase in the volume of the liquid (apparent expansion) is the difference between the actual increase in volume of the liquid (real expansion) and the increase in volume of the container. $V_{observed} = V_{real, liquid} - V_{expansion, container}$ To find the real coefficient of volume expansion of the liquid, one needs to account for the expansion of the container, which itself has a coefficient of volume expansion (or linear expansion, from which volume expansion can be derived). This adds complexity to the measurement process compared to measuring the expansion of a solid rod or block, where the change in length or volume is directly measured. Option D correctly identifies this key difficulty: the container's expansion must be factored in.