What is the correct answer of the Marxes brothers for plural possessive?

The correct plural noun phrase is the Marx brothers.

When using the surname to refer to two or more family members, the surname is singular, functioning as an attributive noun (a noun used as an adjective to describe another noun, also called a noun adjunct). The noun phrase "Marx brothers" is a plural noun phrase, two or more brothers.

The correct plural, possessive form is Marx brothers', indicating that something belongs jointly to the brothers.

Example: The Marx brothers' career began in vaudeville.