<em>Cat</em><em>s'</em><em> personalities and dog</em><em>s'</em><em> personalities are often considered calming to humans. </em>This is the only sentence that has applied the apostrophe rule correctly,
The bold type letters indicate the presence of the apostrophe used to indincate possession. The plural nouns are in the genitive case and when the plural finishes in "s", the apostrophe is put above the word.
These sentences have not applied the rule correctly:
-Cats personalities and dogs personalities are often considered calming to humans. ( Right version: Cats' personalities and dogs' personalities are often considered calming to humans)
-Cat's personalities and dog's personalities are often considered calming to humans. ( Right version: My Cat's personality and my dog's personality are often considered calming to me. If the noun is in the singular, this refers to a particular case. Plural nouns are used to make generalizations).
-Cats personalities and dogs personalities are often considered calming to human's. ( Right version: Cats' personalities and dogs' personalities are often considered calming to humans. <em>Humans</em> is a noun in the plural. It is in the nominative case , not in the genitive case. It is not indicating possession.)
-Cats' personalities and dogs' personalities are often considered calming to humans'. ( Right version: Cats' personalities and dogs' personalities are often considered calming to humans. Humans must not carry the apostrophe because it is in the nominative case.)