Why do we use withheld state income taxes ($2500) PLUS $350 instead of state income tax liability ($2900) plus $350? What is the difference between the two?

Margo has $2,500 withheld from her wages for state income taxes during 2019. In March of 2019, she paid $350 in additional taxes for her 2018 state tax return. Her state income tax liability for 2019 is $2,800 and she pays the additional $300 when she files her 2019 state tax return in April of 2020. What amount should Margo deduct as an itemized deduction for state income taxes on her 2019 federal income tax return, assuming she elects to deduct state and local income taxes?

Select one:

a. $2,850

b. $2,800

c. $2,500

d. $3,150