Add 's.
amazesapologizesbaptizesbrutalizesblazesbreezescrazescriminalizescustomizesdozesenergizesemphasizesfantasizesfreezesgazesgeneralizesglamorizeshazeshomogenizesmazesmemorizesnaturalizesoptimizesprizespopularizesrealizessanitizessizessummarizessterilizestheorizestraumatizestantalizestranquilizestrivializes
It would entirely depend on the specific word's origin. And the vast majority of the nouns ending in u, end in a diphthong (two or more vowels), and the most correct answer in that case would be "add s".
Treat it like an s "He realised with a growing sense of horror that Mr Munoz' suitcase had been run over by a baggage truck"
zaniestzealotzeitgeistzestzestiestzibetzigguratzionistzippiestzitzitheristzithernistzoograftzoologistzoophilistzootomistzymologist
Apostrophe is placed after the letter z in names ending in this letter. Example: Mr. Sanchez' office or Mr. Sanchez's office
apostrophe immediately after the z and before the s: eg Fritz's book
That is Diaz's cat.
Nouns ending in 'z' add apostrophe s to show possession. Example:La Paz's beach is a favorite destination in Baja Calafornia Sur.
glitz
glitz
Nouns ending in -z (common or proper nouns) add -es to the end of the word to form the plural: Mr. and Mrs. Heintz or the Heintzes.The plural possessive form is Heintzes'.
To show possession for a last name ending in "z," you still add an apostrophe and an "s" ('s). For example, for the last name "Gomez," you would write "Gomez's car" to show possession.
Four letter word beginning with Z and ending with T:Zest
Four letter words ending in z include:quizritz
Ersatz.
zone
6-letter words ending in Z include: kibitz ersatz pizazz