Only then had they made / did they make sure ..........

Only then ….. sure that they would be able to buy the house.
a)had they made
b)did they make (Answer Key)
c)will they make
d)do they make

Source: school exam

Hello,
Would you please be kind enough to have look at the above test and tell me if option A (had they made) works?

Thank you
(I have asked this in WR forum but I have not received an answer. I would appreciate your guidance. Thank you)