"Who has many businesses?"
Translation:¿Quién tiene muchos negocios?
Nothing - it should be accepted.
It appears that when the course maintainers added "empresas" as an acceptable answer option, they forgot to add the feminine plural form of mucho. It incorrectly expects you to say "muchos empresas" as a result.
Report it if you encounter it again.
20 July 2018 ""Quién tiene muchas empresas" is still being marked wrong. Duo is suggesting that the correct answer should be: "Quién tiene muchos empresas"! Reported. Duo please fix! Thank you.
When used as an adjective:
(mucho + uncountable noun) mucho desempleo = much/a lot of unemployment.
(muchos + plural noun) muchos perros = many/lots of dogs.
Note mucho can also be used as an adverb and pronoun. More detail in the link below. http://www.spanishdict.com/translate/mucho
Also, "empresa" is a much more common term for a business than is "negocio," at least in Latin America, per sources I checked. And yeah, it is NOT a masculine noun!