A

Step 1: Recall the definition of Poisson's ratio, which is the ratio of the transverse strain to the axial strain in materials under deformation.
Step 2: Understand that solids have a definite shape that allows for deformation under stress, enabling the measurement of strains.
Step 3: Liquids do not have a definite shape; they adapt to the shape of their container instead, thus they do not maintain a constant volume under deformation, making it impossible to measure the axial and transverse strains consistently.
Step 4: Therefore, liquids cannot be assigned a Poisson's ratio.
Hence, the correct answer is A.