Answer: C. GH
Explanation:
Given: Rectangle ABCD is similar to rectangle EFGH.
Side AB is corresponding to EF [First two letters]
Side BC is corresponding to FG [Middle two letters]
Side CD is corresponding to GH [Last two letters]
Side AD is corresponding to EH [First and last letter]
We know that the corresponding sides of the similar figures are proportional.
Therefore, we can see that Side CD is proportional to side GH.