Which one of the following options represent a valid declaration of a backend server with HAProxy?

Which one of the following options represent a valid declaration of a backend server with HAProxy?

A.
host server_x 10.1.0.1:80 cookie server_x

B.
backend server_x 10.1.0.1:80

C.
server server_x 10.1.0.1:80 check

D.
pool server_x 10.1.0.1:80