SPOILER: AAMC Official Guide OG BB #24 - Experimental Procedure Question

Hi ya'll, wasn't a science major so I am a little weak on experimental procedures.

Can someone explain why B is right and what they are talking about regarding visual fields? (The basics please :) )

https://preview.redd.it/mxtylmfa0zx21.png?width=756&format=png&auto=webp&s=ad3bdabbbe5f38e977ff2d6abffbe3b2997843ba