In reply to Raymondo:
Definitely Rupees, but you can't get them until you get there, so take one of the cards here to withdraw from cash machines in bigger towns: https://www.moneysavingexpert.com/credit-cards/travel-credit-cards/
More remote places won't have cash machines, so plan ahead.
The more touristy places may take USD, but the exchange rate will be horrible.
Getting the balance of cash right is tricky, so you have plenty during your trip, but not too much left at the end. Quite a lot of places may take card payments, but the internet can be flaky (like mobile internet in more rural places) and it may not go through - we ended up part paying a couple of bills with USD due to this.
That was 2017, though.