Final answer:
The correct answer is social facilitation, which describes the improved performance of individuals in the presence of others during tasks they are competent at or well-prepared for.
Step-by-step explanation:
The instance described where Heather gives her finest piano performance in the presence of the largest crowd she has ever seen is an example of social facilitation.
Social facilitation occurs when an individual's performance improves because they are in the presence of others.
This phenomenon is particularly relevant when the tasks being completed are straightforward or well-rehearsed, as is presumably the case with Heather's piano performance.