what keyword is required to pass the Host header from the original request to the proxied server?

When trying to reverse proxy a web server through Nginx, what keyword is required to pass the
Host header from the original request to the proxied server?

... {
_________ Host $host
...
}


A.
proxy_pass_header

B.
proxy_forward_header

C.
proxy_set_header

D.
proxy_header