Which of the following taxpayers is exempt from having to file a tax return for $2023?$ a$.$ A $67-$year$-$old surviving spouse with a dependent child and income of $$18,900$ b$.$ Married taxpayers $($ages $45$ and $50$ years$),$ filing jointly, with income of $$29,000$ c$.$ A single taxpayer who is under age $65,$ with income of $$14,500$ d$.$ A $22-$year$-$old student with unearned income of $$2,500$ who is claimed as a dependent by her parents