Note: The abducens n. is actually the nerve most likely to be damaged by an expanding internal carotid aneurysm in the cavernous sinus but they give you specific CN3 function in this question.

Good picture: https://teachmeanatomy.info/wp-content/uploads/Contents-of-the-Cavernous-Sinus.jpg

Question stem says: PE shows Right pupil is larger than the left pupil. (Note: It is not light reflex). And weakness with movement of eye (that means most movements).

CN III palsy - Opthalmoplegia, Ptosis, Mydriasis and loss of accommodation. Ptosis, Mydriasis and loss of accommodation are due to - Parasympathetics travel outside of CN III. So more likely CN III is damaged and not others in the cavernous sinus.

What if you know it's CNIII and remember CN's III, IV, V1, V2, and VI all run through the cavernous sinus, but don't remember which is which in the picture?

CN's IV and VI only control one muscle each and are therefore very small in diameter. V1 and V2 serve a lot of functions and are therefore large. III is intermediate.

Also they are organized in an ascending manner - III above IV above V1 above V2, and VI to the side.

Structures within the outer (lateral) wall of the compartment from superior to inferior: Oculomotor nerve Trochlear nerve Ophthalmic and maxillary branches of the trigeminal nerve Structures passing through the midline (medial) wall: Abducens nerve Internal carotid artery accompanied by the Internal carotid plexus

https://en.wikipedia.org/wiki/Cavernous_sinus