What are the treatment options for a net operating loss occurring in tax years after December $31,2017,$ and before January $1,2021?$

a$.$ Five$-$year carryback and $20-$year carryforward.

b$.$ No carryback and $20-$year carryforward.

c$.$ Five$-$year carryback and indefinite carryforward.

d$.$ No carryback and indefinite carryforward.