Optativo
Facultativo
Potestativo
Some Spanish words that end in "u" include "menú" (menu), "atún" (tuna), and "virtu" (virtue).
flaco
There are none.
Some examples of Spanish words that end in "ir" are dormir (to sleep), abrir (to open), and vivir (to live).
There aren't any Spanish words that end in K. K is not used in Spanish writing, except in a very few loan words from English and other foreign languages.
Libertad and dificultad
yes
jugo (juice)
superávit
"nadie sabe"
Some Spanish words ending in "ad" include: verdad (truth), ciudad (city), libertad (freedom), oportunidad (opportunity).
Perhaps other Spanish words that end with -ar. cantar