Sandra and Christopher were married 18 years ago. They had one child, Noah (13). Noah has no income, and he has never provided more than half of his own support. Christopher passed away in 2020, and Sandra has not remarried. Sandra and Noah lived together all year long; no one else lived with them. Sandra paid all the expenses of maintaining their home. What is Sandra's most beneficial filing status for 2021?

-Single.

-Married filing jointly.

-Head of household.

-Qualifying widow(er).