Eileen, a U$.$S$.$ citizen, died on December $17,$ Year$1.$ The value of her gross estate at the date of death was determined to be in excess of $$12\mathrm{.}92$ million. Assuming the estate chooses to have an estate tax apply, without regard to extensions, what is the due date of Form $706,$ United States Estate $($and Generation$-$Skipping Transfer$)$ Tax Return?

A$.$September $17,$ Year $2.$

B$.$April $15,$ Year $2.$

C$.$December $31,$ Year $1.$

D$.$February $28,$ Year $2.$