(b) The difference of amount of interest between what is charged from borrowers and what is paid to depositors.
Banks charge a higher interest rate on loans than what they offer on deposits; this difference is their main source of income.
Source: Money and Credit, Chapter 3, Loan Activities of Banks
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The passage explicitly states: "Banks charge a higher interest rate on loans than what they offer on deposits. The difference between what is charged from borrowers and what is paid to depositors is their main source of income." Option (b) captures this exactly. Options (c) and (d) confuse "interest rate" with "interest amount" or talk about principal amounts — both are incorrect as per the text.