Google Adsense on WordPress.com

Sure, you could place Google AdSense there but it won’t make you much money.

You’ll need WP self-hosted (wordpress.org) to be taken seriously.

You’ll want to ask this question over at the WordPress.COM support forums — but I’m pretty sure it is not allowed.

WordPress.com is a free blogging tool. It’s good for many purposes but making money isn’t one of them. You’ll want your own domain and more control over your site. That’s where WordPress.org comes in.

Read more about it at: http://en.support.wordpress.com/com-vs-org/.