Determine the [OH−] of a solution that is 0.150 M in CO2−3.
Kb(CO32−)=1.8×10−4

The [OH−] of a solution : 0.0052 M

Kb is the base ionization constant  

LOH (aq) ---> L⁺ (aq) + OH⁻ (aq)  

Dissociation of weak base of CO₃²⁻ :

Reaction

CO₃²⁻ + H₂O⇒ HCO₃⁻ + OH⁻

use ICE method :

I : 0.15               0             0

C : x                   x             x

E : 0.15-x            x            x

i'm assuming that the correct answer is indeed the first one/ "a"..