Default terms based on user role I have Node with taxonomy terms.
Example


Term A 
Term B 
Term C


and user role like this


Role 1 
Role 2 
Role 3


Using Taxonomy Access Control(TAC) I can limit Role 1 can add all terms, Role 2 only add Term B, Role 3 only add term C.

With CSS I can hide disabled options, so when Role 3 want to add content. ex: Role 3 will only see Term C.

But how can I make Term C autofilled when Role 3 want to add. Right now Role 3 must choose or fill two options, nothing and Term C. 

Any ideas?
There might be better ways to do it, but this what I did in my case. 


In hook_form_alter loop through the '#options' for this field.
Store every item in a new array skipping the one you don't want as options for this field.
 (in my case it was the item at index 0 with value 'N/A').
Assign this new array of options as '#options' for the field.


Worked for me.