Question says "On every step of ladder there are $2$ bulbs".
"If one is Green other must be Yellow". So one pair of lights possible = $\color{maroon} {(G,Y)}$
"If one is Red other must be Blue". Other pair possible = $\color{maroon} {(R,B)}$
So, from the above statements, we can conclude that there will be no pair as $(G,B)$ or $(G,R)$ or $(R,Y)$. But, the pair $\color{maroon} {(Y,B)}$ is a possible combination of lights in one of the ladder.
If we consider that there are $3$ steps in ladder as $(G,Y)$ , $(B,Y)$ and $(B,Y)$.
NONE OF THE ABOVE OPTION MATCHES (Official key was D though)