Colonialism had a great impact on the lives of the aboriginal Australians who were eventually (1) _______ by the whites with all power and privilege. The colonisers turned their land into rubbish pits and (2) _______ sites for their own betterment. The aborigines were often (3) _______ as sub-humans with low status and dirty habits. The whites not only displaced the tribes off their homeland but also (4) _______ the beauty and balance of the natural world. An increase in deforestation and destruction of traditional land led (5) _______ like emu, eagle, and kangaroo, among many others to dwindle over time.

Select the most appropriate option to fill in blank number 4.

To find the most appropriate word for blank number (4), let us analyze the context of the sentence in which it appears:
"The whites not only displaced the tribes off their homeland but also (4) _______ the beauty and balance of the natural world."

The sentence uses the correlative conjunction structure "not only... but also..." to connect two parallel negative actions committed by the colonisers:
1. Displacing the tribes off their homeland (a negative action affecting the people).
2. A verb in the past tense that describes a negative action affecting "the beauty and balance of the natural world."

Let us evaluate the given options:
- destroyed: To ruin or put an end to the existence of something by damaging or attacking it. This fits perfectly as it describes how the colonisers ruined the beauty and ecological balance of the environment.
- climbed: To go up or ascend. This is contextually irrelevant to "the beauty and balance".
- utilised: To make practical and effective use of. While colonisers utilised resources, the sentence focuses on a negative impact parallel to displacement, and "utilised the beauty and balance" does not fit this negative, destructive context.
- fed: To give food to. This does not make sense in relation to "the beauty and balance of the natural world".

Therefore, the word "destroyed" is the most appropriate option to fill in blank number 4, as it highlights the negative environmental impact of colonialism that aligns with the subsequent sentence describing deforestation and dwindling wildlife.

Thus, the correct option is destroyed.