C,宾语从句,what was to become the area's chief economic activity,agriculture.
在北美,Nebraska文化承接Woodland文化,大约是在公元1300年,它在作为该地区的主要经济活动的农业方面遥遥领先.
该宾语从句的what 占用的是主语的位置,如果是要用which was,应该要再前面加something,比如pioneered in an area which was to become.
因为是宾语从句必须要用疑问词引导所以A和D选项都不对