Answer:
c. Her mortgage payments and necessities are fixed
Explanation:
Discretionary income is the remaining income after being paid out for all fixed expenses (i.e. Discretionary income = Salary - Mortgage - Income tax etc). The primary reason for variability in it is due to the mortgage payment and fixed expenses from the basic salary received.
So, option c is correct while other options are incorrect as tax does not affect as well as cost of living