Various positions of a prism ABC (with BC as the base) are used. White light is incident on the prism in a narrow beam. Which of the following situations, after dispersion, has the third colour from the top match the sky's colour? 

When the prism is kept with base BC at the bottom, and a beam of white light is incident on the prism, then the emergent band of colour would show violet at the bottom, and the third colour from the top is yellow (see image A below). But we have given the third colour from the top is the colour of the sky, i.e. blue. If the prism is kept with base BC at the top, then violet would be at the top and blue at the third. Thus the prism must be inverted, i.e. the base BC must be on top to obtain a dispersion of light with blue colour at the third position from the top. 

Hence the correct option is (b) ii.