Muslimātun is a nominative subject; ḍamma marks nominative, alif+tāʾ marks plural, and tanwīn is classified as muqābala.
Opening interactive controls...
Nominative Feminine Sound Plural
Tanwīn: Five Types, Their Function, and Omission
Muslimātun is a nominative subject; ḍamma marks nominative, alif+tāʾ marks plural, and tanwīn is classified as muqābala.