@elporko To maybe make what I'm saying more explicit, a better version of answer A would read, "whether or not one can assume that the increase in the number of Mexican Americans born in the United States led to an increase in ethnic Mexican political activism."

Could you not also more simply eliminate answer choice A by saying "Mexican American political activism" =/= "Ethnic Mexican political activism?"

To me, this distinction seemed to be the trap of this question, but I don't see anyone else saying this.

A says, "whether or not one can assume that the increase in the number of Mexican Americans... led to an increase in Mexican American political activism." But the author keeps talking about how you can't extend that to ethnic Mexican political activism.

Quote: "García may be exaggerating the degree to which the views of these activists were representative of the ethnic Mexican population residing in the United States during this period.

...

García argues that between 1930 and 1960 a new generation of Mexican American leaders appeared, one that was more acculturated and hence more politically active than its predecessor. [So, Mexican Americans are more politically active.]

...

one cannot assume that an increase in the proportion of Mexican Americans born in the United States necessarily resulted in an increase in the ethnic Mexican population's political activism." [Or, Mexican Americans being more politically active doesn't mean ethnic Mexicans are.]

To me at least this seems like a much easier reason to eliminate A than talking about whether the author is uncertain if you can assume or not.