The correct answer is (d), adultery.
Lian is a form of divorce in Islamic law that is available to a wife when her husband accuses her of adultery but is unable to provide four witnesses to support his claim. In this case, the wife can demand a divorce and the husband is required to pay her a financial settlement.
The other options are not grounds for divorce by Lian. Option (a), re-embracing her initial faith, is not a ground for divorce in Islamic law. Option (b), cruelty, is a ground for divorce, but the wife would need to provide evidence of the husband’s cruelty in order to obtain a divorce. Option (c), conversion to other faith, is also a ground for divorce, but the wife would need to convert to a faith that is considered to be apostasy in Islam in order to obtain a divorce.