Topic
Explanation:
The topic is the subject analyzed or described in an essay.
In this case, Marina has the task of writing an essay that describes the impact of rock music on American culture, the topic is therefore: the impact of rock music on American culture.
She can describe the topic chronologically or not. For example, Marina could start her essay by reffering to Chuck Berry, Elvis Presley or the Beatles at first, and then advance throught the following decades telling the history of rock and its association with American culture.