Answer:
C). A very important event or achievement.
Explanation:
In the context of the given article, the word 'landmark' denotes 'a notable event or discovery with a historical significance.' Here the author conveys the importance of the act passed in 1975 in the favor of farmworkers as it allowed them to set up unions in order to put forward their demands of a better pay and enhanced working conditions. Thus, the author uses the word 'landmark' to display its historical importance and hence, option C is the correct answer.