Answer: The genitive case is commonly shown in Modern English nouns (3.) adding 's to the singular.
Explanation: The genitive case in Modern English is used to indicate possession, that is to say to indicate that something belongs to someone. The genitive case is shown adding 's to a singular noun. For example, the genitive case of the noun "girl" is "girl's". However, the genitive case of plural nouns is formed only by adding an apostrophe ('). In that way, the only correct option is 3. adding 's to the singular.