I have 5 extra field groups with 30 extra field.
On K2 >> Categories link I want to display only one sceciific Extra Field.
Is it possible to do.
I use Jommla 2.5 and K2 2.6.5

try using this type syntax in either category_item.php or item.php

To get the extra field value you would simply do this:
$this->item->extraFields->EXTRAFIELDALIASHERE->value