What is the subnet mask for the network 172.16.0.0/21?

What is the subnet mask for the network 172.16.0.0/21?

A.
255.255.0.0

B.
255.255.255.240

C.
255.255.248.0

D.
255.255.255.248

E.
255.255.248.240

F.
255.255.255.0