agar123I believe choice D has an ambigous pronoun "it".
D) Once men used to think of land as simply private property, but they consider it now ..
You can read the "it" to be private property or land. Both nouns make sense in the context of the sentence. However, we do not know whether the pronoun refers to land or private property and each usage changes the mean of the sentence.
Once men used to think of land as simply private property, but they consider land (it) now ...
Once men used to think of land as simply private property, but they consider private property (it) now ...
All the other choices with "it" only has land as the only singular noun that makes sense, making it clear that it refers to land. If you read "it" as "a change," the sentence fails to make sense.
In answer choice C, can anyone clarify if you change the sentence to be:
C) There has been a change in how men think of land, a change from once considering it "as" simply private property to now considering it
or
C) There has been a change in how men think of land, a change from once considering it simply "as" private property to now considering it
Would this make C correct? I fail to see a reason to eliminate it.