The concept mentioned is Fractional Reserve Banking.
1) Option A, banks hold a fraction of deposits in reserve.
2) Option A, guarantees that depositors will always get their money, avoiding bank runs.
Fractional-reserve banking is the system of banking operating in nearly all countries worldwide, under which banks that take deposits from the public are needed to hold a proportion of their deposit arrears in liquid means as a reserve, and are at liberty to advance the remainder to borrowers.
With a fractional reserve banking system, a fall in bank reserves results in multiple cascades in demand for deposit money. Deposits are insured by the Federal Deposit Insurance Corporation.
In a system known as fractional reserve banking, only a portion of bank deposits are guaranteed by factual cash that's on hand and accessible for withdrawal.
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