(b) In accordance with...
is like saying Following...
Following your instructions, we have closed your bank account.
I think this is the intended meaning in context, as you generally have the right to close a bank account.
(a) With regard to...
is like saying Considering...
This would be OK if the result might not be as instructed, but not in a case where you expect your instructions to be carried out.
With regard to/Regarding/Considering your request for a million dollar loan, unfortunately we cannot help you.