What is the feminine of sultan?

In English there are no masculine or feminine forms. English uses gender specific nouns for male or female.

The title for a male is sultan; the title for a female is sultana (not an equivalent role as a sultan, but defined as the wife, concubine, or female relative of a sultan).

It is highly unlikely that a country ruled by a sultan would permit a female head of state. However, in cases where there is no male heir, a daughter or wife may succeed as a Sultana or Maharani. The people may not approve of a female ruler in a Muslim kingdom that its Council of Elders may later replace her for a chosen suited Sultan. A Sultana may not last for long as the people may find one suited as as their Sultan. It depends, if one may cause the edification of her country State being the Sultana who may have the country convincing power to lead.

If you speak Urdu, then Sultan means king and the feminine form would be Mallika.