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$\textbf{First Method:}$

Given that,

- One litre of liquid $1$ weighs $ = 1 \; \text{kg} = 1000 \; \text {gm}.$
- One litre of liquid $2$ weighs $ = 800 \; \text{gm}.$
- And half litre of mixture weighs $ = 480 \; \text{gm}.$

When both liquids given and mixture also given, we can use the alligation method.

Half litre of mixture weighs is $ 480 \; \text{gm}.$

Therefore, the weighs of liquid $1$ and liquid $2$ also become half.

Now, we can apply the alligation method.

$\therefore$ Percentage of liquid $1$ in the mixture $ = \frac{4}{5} \times 100 = 80\%.$

$ \textbf{Second Method}:$

$1$ litre mixture weigh is $480 \times 2 = 960 \text{gm}.$

$\therefore$ Percentage of liquid $1$ in the mixture $ = \frac{4}{5} \times 100 = 80\%.$

Correct Answer $: \text{D}$