Which of the following sentences contains a linking verb?

i did not have to go to work today.
tara’s mother is a lawyer.
stephanie came over after she finished her homework.
miguel and sarah studied together at the library.

Liking verbs are verbs that connect with the subject and give some information about it. Example: am, is, appears, look, seem, taste, etc...

Linking verbs don't express action. But, as usual, we have the famous exceptions. Some verbs can be action verbs or linking verbs, example: 

grow, look, prove, remain, smell, sound, taste, turn, stay, get, appear, feel

So, answering you, the answer is the second sentence.

China plays a major role in population and china is also a well developed country