I believe the correct answer from the choices listed above is the second option. The expression (6.0 x 104) (3.1 x 10-1) is equal to 1.9 x 10^4. We only have two significant figures from the starting expression that is why we rounded of the product from 1.86 to 1.9.
I think the correct answer from the choices listed above is the second option. When two hydrogen atoms enter the ETS as part of either NADH or FADH2, the two hydrogen atoms are split into two H+ and two electrons. Hope this answers the questions.
