Answer
a. and c.
Work Step by Step
Find the smallest integers numbers you can get by dividing all atom subscript values by the same number.
a. $C_2H_2$ divided by 2: $CH$
$C_6H_6$ divided by 6: $CH$
b. $C_3H_6$ divided by 3: $CH_3$
$C_4H_{10}$ divided by 2: $C_2H_5$
c. $NO_2$ cannot be lowered: $NO_2$
$N_2O_4$ divided by 2: $NO_2$
d. $C_{12}H_{10}O$ cannot be lowered: $C_{12}H_{10}O$
$C_6H_5OH$ cannot be lowered: $C_6H_5OH$
