Why did migrant farm workers on strike in California in the late 1960s ask people to boycott grapes? A) The grapes were picked by immigrants who...

This is a very difficult question to answer because both C
and D appear to be plausible answers.  I would suggest that you look in your textbook or
in your notes to see if there is an answer that your book/teacher says is the right
one.

I would argue that D is the best answer here.  Cesar
Chavez's movement was certainly meant largely to create a union that would represent
migrants who did the manual labor on farms.  However, the point of boycotting grapes was
to put pressure on the growers to meet the demands of Chavez and his
movement.

So, either C or D really could be correct. 
Chavez wanted to create a union, but the point of the union was to get better working
conditions and wages for the workers.