Answer
In b. the pair of compounds have the same empirical formula.
Work Step by Step
1. Identify each empirical formula.
a. $H_2O$: Cannot be divided.
$H_2O_2$ divided by 2: $HO$
b. $N_2O_4$ divided by 2: $NO_2$
$NO_2$ cannot be divided.
c. $CO$ cannot be divided.
$CO_2$ cannot be divided.
d. $CH_4$ cannot be divided.
$C_2H_6$ divided by 2: $CH_3$
