a$.$ When actual inflation exceeds expected inflation, debtors gain at the expense of creditors because they repay their loans with depreciated currency.

b$.$ When expected inflation exceeds actual inflation, debtors gain at the expense of creditors because they repay their loans with depreciated currency.

c$.$ When actual inflation exceeds expected inflation, creditors gain at the expense of debtors because they repay their loans with devalued currency.

d$.$ When actual inflation exceeds expected inflation, debtors and creditors both lose because they repay their loans with depreciated currency.