In 1859, two dozen rabbits were introduced to australia. by 1953, there were an estimated one billion rabbits in australia. the addition of the rabbits to this established ecosystem caused many problems, including a huge decrease in native plants. this left little food for native animals. this is an example of how
a) diversity improves an established ecosystem.
b) genetic variation leads to the formation of a new species.
c) the introduction of non-native species upsets a balanced system.
d) changing in one part of a system does not affect other parts of the system.

Here's an unorthodox approach that you might try: 
READ each choice and think about it for a coupla seconds !

A) diversity improves an established ecosystem.
No.  The results were not an improvement.

B) genetic variation leads to the formation of a new species.
No.  There's no mention of genetic variation or new species in the story.

C) the introduction of non-native species upsets a balanced system.
YES !  That's exactly what is described in the story.

D) changing in one part of a system does not affect other parts of the system.
No. A very great effect on the system is described.