A、on has taken
the farm as well as its neighboring hills we once spent so much time _on has taken_on a new look
以下成分分析兼做解释.
the farm as well as its neighboring hills we once spent so much time _on主语,其中we once spent so much time _on为定语从句
has taken _on谓语
a new look 宾语
选C/D的话,主句缺乏谓语；而B,则语法不对,因为spend time on sth,而不是spend time sth （the farm as well as its neighboring hills we once spent so much time _on中省了which=the farm as well as its neighboring hills [which] we once spent so much time _on=the farm as well as its neighboring hills [on which] we once spent so much time _）