Power. "My Last Duchess<span>" is all about power: the political and social power wielded by the speaker (the Duke) and his attempt to control the domestic sphere (his marriage) in the same way that he rules hi...</span>
The sentence that has the correct subject- verb agreement is the following one: Neither the Western Hemisphere nor the Eastern Hemisphere are immune to flooding.
The verb "to be" can't be singular because the writer mentions the two hemispheres (the Western Hemisphere and the Eastern one). That is why the first option is the correct one.