Which of the following words would best describe something that is thick and difficult to pour?

The word that best describes something thick and difficult to pour is "viscous." This term specifically references substances that have a high resistance to flow, meaning they are thick and not easily pourable. Substances like honey, syrup, and molasses are common examples of viscous materials.

In contrast, "aqueous" pertains to a solution in which water is the solvent, but does not inherently denote thickness or difficulty in pouring. "Fluid" describes a state of matter that can flow and does not imply anything about thickness, as it can refer to both thick and thin liquids. "Watery" characterizes a substance that is thin and has a consistency similar to water, which is the opposite of being thick. Therefore, "viscous" is the most appropriate choice for something thick and difficult to pour.