To enable reCaptcha in Drupal we have to enable the reCapctcha module.

Type 'drush en -y recaptcha' on Terminal.

This will download and enable the recaptcha module and its dependencies such as Captcha module.

Then goto admin/config/people/captcha/recaptcha.

There we have to give the 'Site key' and 'Secret key' under the General settings.

By clicking the link below the text input, goto www.google.com/recaptcha/admin#list.

Generate the Site key and Secret key by giving the domain name and site name in the Register now section.

Use the both keys in the General setting of reCaptcha section tab.

We can also set the other widget settings in the same tab as users need.

Then click the Captcha tab. (admin/config/people/captcha)

There change the default challenge type to 'reCaptcha(from module reCaptcha)'.

Then give the form ID of the form and select Default Challenge Type.

Below we can find the check boxes to show the reCaptcha in admin pages also.

Enable the check boxes as our needs and click save configuration.

We can find the reCaptcha in the Drupal form.