How to add reCaptcha while doing comments on my drupal website
Thank you all, Its done. Procedure as follows :

I have downloaded 2 modules CAPTCHA &amp; reCaptcha. 

Then extract them into /var/www/site/modules/ . 

Then I have restart my web server. Then goto browser.

Next I have found CAPTCHA and reCaptcha options on modules section. 

Then I configured CAPTCHA settings in configuration section according to my requirement.

Thats it !!  When I try to do comment on my website it was given me captcha challenge This is what all i need. 

Thanks!! :)