JW 05/14 daily CP passage Q3

my logic: there's excess carbon monoxide, making the limiting reagent hydrogen gas...to my understanding, the reaction under these conditions would depend only on the concentration of hydrogen gas (1st order), thus eliminating all the other options.

C/P is inarguably my weakest section so i was reading up on the kaplan gen chem book & it says that rate law coefficients are determined experimentally. again, i used the logic described above because this question lacked experimental data. the book also said that stoichiometric coefficients are rarely (not never... but still rarely) the same as the rate law orders, but that is the logic JW used in their explanation.

is there something i'm missing here? i'm thinking it might have something to do with the "elemental reaction" part of the question, but even after some googling, it's still unclear to me when i would differentiate this case from other cases where coefficients ≠ rate law order...