It's a way to select by default N/A checkbox I want that if a fields is not mandatory the N/A options on select box is ticked by default.

I have tried to tick default value N/A on the field but after the setting disappear.

I use Drupal 7.15.

Cordially
The default N/A will comes the field is not required(not mandatory) and the value N/A will enter to database enen if you are not selected anything.

If you want a N/A selected . Enter another value named "N/A" and make the field as "Required"
and mark the default value as "N/A"(The new N/A will appear along with the default N/A).

Hope this will helps you