Final answer:
Cesar Chávez's central purpose in his text was to encourage a boycott of grapes to improve labor conditions for farm workers, articulated through the slogan "Sí se puede" or "Yes, it can be done!"
Step-by-step explanation:
The central purpose of Cesar Chávez's text is to inspire and mobilize the public to boycott grapes, a key strategy used by the United Farm Workers (UFW) to advocate for the rights of farm workers and improve labor conditions. Although Chávez's speech itself is not provided in the question, information given indicates that the boycott was effectively articulated through the slogan "Sí se puede" or "Yes, it can be done!" This would likely be the part of the speech where Chávez addresses his central purpose, calling on consumers to use their buying power to force changes in the agricultural industry.