The thickness or density of liquid is due to viscosity, a fluid property of every liquids used in preparations. This is responsible for adhesion or cohesion properties of liquid.
The zygote (formed after fertilization) or sublingual (under the tongue) are not going to depict the property of thickness of a liquid.
Thus, the options (b), (d) and (c) are incorrect.
The thickness or density of liquid is due to viscosity, a fluid property of every liquids used in preparations. This is responsible for adhesion or cohesion properties of liquid.
Therefore, the correct option is – a. viscosity