What variable tag must be included in an email to allow prospects to manage their email preferences?

What variable tag must be included in an email to allow prospects to manage their email preferences?
A . %%opt_out%% or %%email_preference_center%%
B . %%unsubscribe %% or %% email_preference_ center %%
C . %%opt_out%% report_spam %%
D . %%unsubscribe % or %% opt_out %%

Answer: B