How to access key and value of an array in smarty template?

Use key in your foreach:

{foreach from=$enquiries_labels item=label key=key}
          <option value="{$key}" {if $data.key == $key} selected="selected" {/if}>{$label}
          </option>
{/foreach}

It's all there in the documentation.

Smarty 3 foreach construction is like this

{foreach $products as $p}
    {$p@key}: {$p}
{/foreach}

Use the key attribute.

{foreach from=$enquiries_labels item="label" key="key"}
    <option value="{$key}"{if $data.key == $key} selected="selected"{/if}>{$label}</option>
{/foreach}