relations...

i think she was a friend of Nora and he was of Helmer, old friendship i guess

they were old lovers but she chose to marry someone else for money so she could support her mother. check out act III, they had agreed to get married.

(A Doll’s House) Mrs. Kristine Linde knows Krogstad because:

They used to date

He is her employer

They work at the same bank

They were married

The answer is they used to date