Why do you have "root" there? It should not be necessary to specify the username, this will attempt to execute the command root /usr/bin/php /var/www/html/cron.php
i guess problem will be in php location /usr/bin/php, open terminal and run the command /usr/bin/php /var/www/html/cron.php do you see any error message ?
root /usr/bin/php /var/www/html/cron.php$Cronisn't clearly defined./usr/bin/php, open terminal and run the command/usr/bin/php /var/www/html/cron.phpdo you see any error message ?