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A pure substance is a material that is made up of only one type of particle (element or compound) and has uniform properties throughout. A solution is a homogenous mixture (a mixture that has the same composition throughout) of two or more substances (e.g., sugar water). A heterogeneous mixture is a mixture with a non-uniform composition, meaning that the different components of the mixture can be visually distinguished (e.g., sand and water).

A gold coin is typically made of solid gold throughout its entirety. Although there might be some minor impurities or other metals mixed in, we will assume that it is a pure substance because the gold content is predominant.

A cup of coffee is a liquid that is the result of combining water with dissolved coffee solids (and possibly sugar, milk, or other additives). Although the solids may come from various sources, making the mixture non-uniform at a molecular level, a cup of coffee appears to be the same substance throughout its entirety. Therefore, we could classify it as a solution.

A wood plank is made from the fibers and natural compounds found in a tree. It contains multiple components, such as cellulose, lignin, water, and other substances, that may not have a uniform distribution throughout the wood. As such, we can classify the wood plank as a heterogeneous mixture.

The descriptions of the items in the question leave some room for interpretation, which could affect their classifications: - For the gold coin, no mention is made of its purity or whether it is mixed with other metals. A gold coin made from an alloy of metals would be considered a solution, not a pure substance. - In the case of the cup of coffee, potential variation in the amount or source of the coffee solids might affect our decision on whether or not to classify it as a solution. Similarly, the presence of additives like milk or sugar could change the uniformity of the liquid. - Lastly, the description of the wood plank does not provide specific information about its composition or the degree of variability in the distribution of its components, which could also influence our classification.