Which of these is a pronoun? a. they b. who c. that d. thiers.

Pronoun is a word that can replace a noun, for example: I, you, he, she, we. 
For example, instead of saying, The teacher talked, you can say "She talked"
"They" is a pronoun because it can replace a noun such as the teachers.
Instead of saying The teachers talked, you can say "They talked." 

"Who" is also a pronoun as it can act as a subject. 
You can say: "Who is this man?" and who is a pronoun (specifically interrogative pronoun).