<?php
$db = JFactory::getDBO();
$id = (int) JRequest::getInt('id');
$db->setQuery("SELECT c.name AS name, c.alias AS slug FROM #__k2_items AS i INNER JOIN #__k2_categories AS c ON i.catid = c.id WHERE i.id = {$id} AND i.published = 1");
$category = $db->loadResult();
echo "$category->name";

<?php
$db = JFactory::getDBO();
$option = JRequest::getCmd('option');
$view = JRequest::getCmd('view');
$id = JRequest::getInt('id');
if ($option == 'com_k2' && $view == 'item') {
    $db->setQuery("SELECT c.name AS name, c.alias AS slug FROM #__k2_items AS i INNER JOIN #__k2_categories AS c ON i.catid = c.id WHERE i.id = {$id} AND i.published = 1");
    $category = $db->loadObject();
    echo "$category->name";
}