What you see as the most significant barrier that immigrants face in attaining a successful, comfortable, and/or peaceful existence in the U.S. Race? Class? Ethnicity? Gender? Age? Stubbornness? Bad luck? What solutions, if any, does the author of the text you have selected present in aiding immigrant characters from whom successful, comfortable, and/or peaceful existences in the U.S. are withheld?

The first question here can be answered from a number of different angles. Here are some ways to approach the question that might be helpful.
The most significant barrier that immigrants to America face really depends on where they are coming from and where in America they are going. One way to construct a response to this prompt involves thinking about which specific immigrant group you would like to learn about. Boyle's Tortilla Curtain concerns the plight of Mexican immigrants in California, so that could be a good place to start. Additionally, if you come from an immigrant family yourself, you may want to find out more about your own family's story so you have something to compare to the experience of Cándido and América Rincón.
All of the barriers suggested by the prompt are valid, but some barriers are more difficult than others depending on where in America an immigrant chooses to settle. Consider thinking of each option from your own point of view: imagine yourself moving to a different country. What kinds of problems might you encounter? You might also want to think about some of the places in America where immigrants from certain countries settled in the past; for example, Boston has a very large population of Irish Americans, so someone coming from Ireland today might have an easier time in Boston than in San Diego, where fewer familiar accents and customs exist.
Because you have so many options and directions in which you can go to create a credible response, taking some time at the beginning to free-write and to outline is important. Good luck!