For which one of the following countries, is Spanish not an official language?

Chile, Colombia and Cuba are all listed among the sovereign states where Spanish holds official or national status; common lists of Spanish‑official countries explicitly include Chile, Colombia and Cuba and note that Spanish is official across most of Latin America (with Equatorial Guinea the only African country where Spanish is an official language). The Republic of the Congo (Congo‑Brazzaville) is not included among countries that recognise Spanish officially; its official administrative language is French, reflecting its Francophone history. Hence, the country from the options where Spanish is not an official language is the Republic of Congo (option 3).