Imagine That an Alarm Caller Sacrifices Its Life to Save

Imagine that an alarm caller sacrifices its life to save other conspecifics. According to Hamilton's rule, how many full siblings would it have to save for the behavior to be favored, and why?

A) exactly one, to replace its own genes in the gene pool
B) two, because this equals its own genes plus additional genes to make it worth it
C) three or more, because Hamilton's rule is an inequality
D) exactly three, to replace its own genes in the gene pool most efficiently