Class 'WP_Query' not found error


I have created a custom php page and placed it in the root folder of my wp installation

What I’m trying to do is to display 10 of my posts per page (all posts not just testimonials as I have)
I’m also getting an error:
Fatal error: Class ‘WP_Query’ not found in /home/content/47/7945147/html/kath2.php on line 12

I have attached my php code