To add to the great ideas of the first answerer, I thought I would further expand upon your question of "how" to answer in the form of an essay.  Quite honestly, your professor did a great job making it easy for you to write a five-paragraph essay in a definite form!  The original question is a perfect thesis statement.  I would begin the introduction with a current example of a book/film that shares similar themes.  Of course, end your introduction with the thesis.  The three body paragraphs are nicely indicated there:  one for false hopes, one for thwarted desires, one for broken dreams.  Using The Grapes of Wrath it should be very easy to find three examples of each.  Finally, in the conclusion (after you reword your thesis), I would focus on the possibility of hope, especially through the very moving image that ends the novel, . . . one that possibly turns a very tragic situation in to one with hope for the future.

I think that the best approach to such a topic would be to identify specific characters in the story.  Defining the primary motivation of specific characters, such as the Joad family or its individual members, might be a good start here.  Identify where their hopes lie and how these materialize through the course of the novel could help to illuminate this theme with textual support.  In the end, the idea has much to develop as support because there is the idea that broken dreams and deferred desires and hopes were critical in defining the time period and those who had the misfortune of enduring through it.  In any period where poverty, dispossession, and social and emotional pain is so present, the experiences identified in the topic are highly evident.  Linking this theme to individual experiences and narratives will help bolster it.