The word spelling refers to the correct form for writing words which mainly depends on letters that compose a word and the order in which these should be placed to be grammatically correct. Common spelling mistakes can occur when letters in a word are omitted or placed incorrectly. Additionally, to this, spelling is related to many other grammatical elements including the use of singular and plural nouns, which in most cases are formed by adding "-s" or "-es" at the end of words but changes when words derived from other languages.
This is the case of the word "alumnus" derived from Latin, which means a person who has graduated from a school, university or other educational institution and whose plural is "alumni". Considering this, the correct spelling for the word that completes the sentence is "alumni" , because in the sentence this word is linked to the pronoun "they" which is a plural and therefore the noun should be also plural, in this case "alumni" is the plural of "alumnus" and therefore the sentence would be "They are alumni of Notre Dame University".
