The way to form possessives in English is to add ‘s to regular singular nouns and noncount nouns and irregular plural nouns not ending in s or to add an apostrophe after the s ending of regular plural nouns and after singular/noncount nouns ending in the sound /s/ to form ‘s.
Celse-Murcia (2013) affirms that, besides possession, the genitive form can indicate: 

The sentence that correctly indicates an example of a genitive case is:

Select the option that properly expresses possession:

Observe a sentença: “Tesla's interest in electrical invention was spurred by his mother”. Diante das assertivas, assinale a alternativa correta em referência ao apóstrofo ‘s:
I. É a contração do verbo to be na 3ª pessoa do singular (he is = he’s). II. Faz parte do caso genitivo ou caso possessivo, ele indica que algo pertence (está associado) a alguém ou a uma coisa. III. É a contração do verbo to have na 3ª pessoa do singular (he has = he’s). 

In which of the following sentences the ‘s is used with the same function as in the sentence “We wanted to make a song that made sense in the world and in people's lives” (l. 26-27)?

According to the text, judge the item.

The “‘s” in “people’s lives” (line 7) indicates the possessive or genitive case.

Analyze the sentences according to structure and grammar use.
1. The (‘s) in “The Amazon is often called the lungs of the earth and produces 20% of the world’s oxygen.”, is the contraction for the verb to be (is).
2. In “In order to work in the park, conservationists need a reliable map. However, no map existed, and they didn’t have enough knowledge to make one on their own.”, the underlined pronouns refer to ‘conservationists’
3. The phrasal verb ‘cut down’, in the second paragraph, means to reduce the amount.
4. The negative form of “They came up with the idea of involving local tribes to help them, combining modern and ancient methods to produce a map.” is They didn’t come up with the idea of involving local tribes to help them, combining modern and ancient methods ... Ver mais