DELIBERATIVE is a 12 letter word that starts with D.

1. \De*lib"er*a*tive\, a. [L. deliberativus: cf. F.
   d['e]lib['e]ratif.]
   Pertaining to deliberation; proceeding or acting by
   deliberation, or by discussion and examination; deliberating;
   as, a deliberative body.
   
         A consummate work of deliberative wisdom. --Bancroft.
   
         The court of jurisdiction is to be distinguished from
         the deliberative body, the advisers of the crown.
                                                  --Hallam.
   
   

2. \De*lib"er*a*tive\, n.
   1. A discourse in which a question is discussed, or weighed
      and examined. --Bacon.
   
   2. A kind of rhetoric employed in proving a thing and
      convincing others of its truth, in order to persuade them
      to adopt it.