Correction of the city's name: Açu. 

Explanation:
The Orthographic Vocabulary of the Portuguese Language (VOLP), which establishes how words in the language are spelled, recognizes only the spelling "Açu". Brazilian Institute of Geography and Statistics (IBGE), the national agency responsible for the production, analysis, research, and dissemination of geographic, statistical, demographic, and socioeconomic information, also lists the city’s name as "Açu".

According to current Portuguese conventions, toponyms derived from Indigenous words must follow the same spelling rules (transcription into Portuguese) as the languages in which they were created. The term "açu" is the Portuguese version of the Tupi suffix "-usu", which forms the augmentative degree. More precisely, the city’s name originates from the term "taba-açu", something like “large village,” used by the native peoples who inhabited the region to refer to the large number of settlements in the area. Some other words in Portuguese include "jacaré-açu" (large alligator) and "capim-açu" (large grass).

The term "-gûasu" is a variant of "-usu" (in Portuguese: "-guaçu") and also appears in several words in the language. One of them, "Ipanguaçu" (large island), is the name of a city neighboring Açu.

There is also a third spelling of the city: "Assú". This is the worst version because, in addition to what has already been explained, it contains an orthographic accentuation error, since in Portuguese paroxytone words ending in -u ( /a'su/ (as‧sú) ) are not accented.