This dog is hers.句中用hers' 或用 B. her's。 ?

Hers 是一個 possessive adjective，不用加 '。
——————————————————————————————————————
英文一共有七個 pronoun（代名詞）：I, you, he, she, it, they, we例如：I am a boy, she is a girl, etc.

換成 possessive pronoun 後是：my, your, his, her, its, their, our
例如：This is my pen, that is his ball, etc.

換成 possessive adjective 後是：mine, yours, his, hers, its, theirs, ours
例如：This pen is mine, that ball is his, etc.

以上的全部都不須加 '。
——————————————————————————————————————
在這一句，hers 是一個 possessive adjective。
This dog is hers，意思是：這隻狗是她的。
——————————————————————————————————————
當要講的對象不是 pronoun，而是 noun（名詞），便要加 's。
例如：This dog is Mary's，意思是：這隻狗是馬莉的。

當這個名詞最後一個字是 s，便須在字末加 '。
例如：These balls are the students'，意思是：這些球是學生們的。

her, determiner, is the possessive form of she of or belonging to a woman or girl who has already been mentioned or is easily identified:-This is her dog.-----possessive
This is her dog------pronoun objectiveThis dog is hers--------possessive