The correct answer is option C, that is "Spain attempted reforms, which proved largely ineffectual, to help the condition of the Native Americans."
Nonessential information is tipically expressed between commas. The purpose of the commas is to make the reader realize that it is an aside comment. In this case in particular, it is clear that the essential information is that Spain tried to help the native Americans, but not the fact that they were unsuccessful. Notice how the message is lost if you consider that helping the Native Americans is the nonessential information.