ranjeet75 wrote:[spoiler]why B is wrong and in A how "...., to what had been ......." is right?[/spoiler]

At the time of the Mexican agrarian revolution, the most radical faction, that of Zapata and his followers, proposed a return to communal ownership of land, to what had been a pre-Columbian form of ownership respected by the Spaniards.

Use of "to what had been..." is correct in the original choice. "what had been a pre-Columbian form of ownership respected by the Spaniards" describes "communal ownership of land". Use of past perfect is correct here because this communal ownership of land existed in the time of pre-Columbian period, which is of course older than the period when the faction of Zapata and his followers "proposed" to get back to this form of ownership. We need verb sequencing here to distinctly show the different time periods in the past when the proposal was made and when the communal ownership was actually practiced. Indeed, we do not come across such constructions quite often and hence get confused.

Choice B is incorrect because:
1. use of "a form of ownership of the Pre-Columbian" seems to suggest that somebody owned pre-Columbians. The phrase does not communicate the sense of the form of ownership belonging to the pre-Columbians, as does "the pre-Columbian form of ownership". It rather conveys the sense of a form of owning pre-Columbians.

2. Also if you notice, use of "and" introduce a list in the sentence with entities that modify "communal ownership of land". Both the entities in the list are showing the characteristics of this "communal ownership of land" but grammatically they are not parallel. "a form of..." is a noun phrase while "respected..." is a verb-ed modifier.

Hope this helps.
Shraddha