Which sentence uses brackets correctly? the answer is c
a a deduction reduces the amount of your [income] that is subject to tax, thus generally reducing the amount of tax you must pay.
b [even if you are not required to file an income tax return], you may choose to file a return if you are eligible for a refundable credit.
c see form w-4 [employee’s withholding allowance certificate] to determine if you can claim exemption from income tax withholding.
d your status as a full-time student exempts you [from filing federal taxes] if your earnings are less than the filing requirement for your age.