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Explanation:

Efficiency of online display advertising expenditure on

$\mathrm{Website} \mathrm{A}=\frac{2700}{27\% \mathrm{of} 557000}=\frac{10}{557}$

Efficiency of online display advertising expenditure on

$\mathrm{Website} \mathrm{B}=\frac{1200}{22\% \mathrm{of} 557000}=\frac{5.45}{557}$

Efficiency of online display advertising expenditure on

$\mathrm{Website} \mathrm{C}=\frac{900}{18\% \mathrm{of} 557000}=\frac{5}{557}$

Efficiency of online display advertising expenditure on$$

$\mathrm{Website} \mathrm{E}=\frac{1600}{20\% \mathrm{of} 557000}=\frac{8}{557}$

Advertising budget was spent most efficiently on Website A.

Hence, option (a).