the answer is c.
owing the bank means she has a NEGATIVE balance. therefore, choices (a) and (d) are wrong.
(b) is negative, but the absolute value of -20.75 is 20.75. this would mean that she owes the bank 20.75, but this cannot be right as that is more than 20.
(c)'s absolute value is 19.75, which is a little less than 20.