what you have said is is true, but all those who have loans (mortgages etc), will be paying less interest rates, monthly, therefore there will be less money withdrawn from their account on a monthly basis, thus leading to an increase in national savings.* All other things being equal, they don't necessarily increase spending since there has been no mention of an injection of funds into the economy by the government. But part of what your saying is true so there is a chance they could accept both answers.
I think you're getting the two accounts mixed up...
Lets simplify the situation - there are both BORROWERS and SAVERS in the economy (of course you can be both but this is just to make it easier to understand)
Those who BORROW money have an account where they must repay reservicing costs every month/whenever paid WITH interest. These accounts are not SAVINGS accounts, rather, they are accounts that have an amount owing. So if the rates decreased... it will only decrease the amount owing (I don't think these accounts pay you interest lmao).
As these are not savings accounts.. by decreasing the amount owing, it will NOT increase national savings. (unless they decide to put it into a savings account.. but from all things being equal we can't assume that)
Money gained from savings accounts are used to by banks/other financial institutes to lend money to borrowers, and the interest + amount owing paid by borrowers are used to provide savers with interest.
If you look at it the other way around, if you decrease interest rates, this will decrease the amount owing, thus less money is added to savings accounts which will decrease the potential level of national savings.