Strangely they haven’t offered the obvious alternative : ‘ Chavez’s ‘ The other options offered - a, b and d - are all incorrect ; b and d because there is no possessive apostrophe, and ‘a’ because it refers to one person, not several or many. This particular point of written grammar is one which English native speakers frequently get wrong, most commonly by putting the apostrophe before the ‘s’ in a plural word, thereby turning it in to a singular. Of the choices available to you, only ‘c’ is correct, although many people would also accept, and indeed prefer, the missing option : Chavez’s

Hi Check this: https://en.wikipedia.org/wiki/English_possessive "In the case of plural nouns ending in -s, the possessive is spelled by only adding an apostrophe and is pronounced the same (for example: Peasants' Revolt). In the case of singular nouns ending in -s (or -z in another sibilant -z or -x sound; or -se, -ze, -ce or -xe. Example, Verreaux's eagle),[citation needed] the possessive was traditionally[2] also spelled by adding only an apostrophe (despite often being pronounced differently)." Regards

Chavez's is how I was taught in school.