Final answer:
A bar of gold is an example of a pure substance because it is an element with a constant composition throughout, unlike mixtures which have variable compositions.
Step-by-step explanation:
An example of a pure substance is d) A bar of gold. Pure substances include elements and compounds, and they are materials with a constant composition having the same properties throughout the sample. Gold is an element, one of the more than 118 known elements, and it cannot be broken into simpler substances by chemical changes, making it a pure substance. Other examples like a glass of juice, a bowl of cookies, and a bag of nuts are mixtures because they contain more than one substance with variable composition.