I am much confused to use whether 'Lac' or 'Lakh', So which word is correct to use? (In terms of money)
I understand your confusion.
At least in India, both are in practice. However, if you typically go by its Hindi origin 'लाख', the spelling 'Lakh' is proper.
It's worth noting that OALD has an entry for 'Lakh' and not 'Lac'.
[Personally, I prefer/practice 'Lakh'].
Though both spellings are in use, Lac also means a resinous material.Preferably we should spell it as Lakh which has no other meaning.