Is Tess a pure woman? If yes, how? If not, why not?
Clearly, Hardy believes that Tess is a pure woman. He says so in the subtitle to the work and he often refers to her as pure or innocent. So the author clearly sees Tess's purity.
Scholars argue over whether Tess truly is pure. Those who think she is not pure point to the fact that she sleeps with more than one man in the book, not always while married. In addition, she kills a man. Both of these are, arguably, signs of someone who is not pure.
On the other hand, some scholars point to the idea that she does not act out of any motives that are impure. Her worst actions come when she lets Alec have her body and when she kills him. But in both cases, she is only doing it because of her desperate situation.
So an argument can be made both ways and you should decide which argument is more compelling to you.