how to show html content when select box item selected i have a select box called Delivery method and it has two options Drop off and pick up..,i want to load few extra html fields like size ,pick up address to the page when user select pick up method,what is the best way of doing it drupal..
In custom form you can use ajax API for this. You can find the sample code from here

When you are using node form then use conditional_fields module for this purpose.