Final answer:
A material fact in real estate is a piece of information that affects a property's value or desirability. In the given question, 'The existing roof is 20 years old' is a material fact, as it is significant and verifiable for potential buyers, unlike subjective opinions.
Step-by-step explanation:
Among the options provided, the example of a material fact would be 'The existing roof is 20 years old'. A material fact is information that would be significant to a reasonable person in deciding whether to engage in a particular transaction. In real estate, material facts are those that affect the value or desirability of a property, and they must be disclosed to potential buyers. Unlike opinions such as 'The house is the nicest house on the street' or 'The street is the best street in the city', the age of the roof is a verifiable piece of information that may impact the maintenance and insurance costs, as well as the overall living experience for potential buyers.