How can I display dateUpdated for the most recent updated entry in a category? I would like to display the date that the site was last updated. This date is the same as the last entry added to a specific category. Thanks in advance.
You can order entries by dateUpdated in descending order and then get the first entry. You would use that entries dateUpdated field to show when the site was last updated.

{% set entry = craft.entries.order( 'dateUpdated desc' ).first() %}
{{ entry.dateUpdated }}