When two figures are similar, the ratio between corresponding parts of the shapes is the same.
In this case, the segment BC corresponds to GH and the side HI corresponds to CD. Then:
[tex]\frac{CD}{HI}=\frac{BC}{GH}[/tex]
Replace HI=49, BC=3 and GH=21. Then, solve for CD:
[tex]\begin{gathered} \Rightarrow CD=\frac{BC}{GH}\times HI \\ \\ =\frac{3}{21}\times49 \\ \\ =7 \end{gathered}[/tex]
Therefore, the lengh of side CD is 7. The correct choice is option d) 7.
