Problem #51- page 115 in Bruice
In order to solve this question, we need to define if the substituents are on the same side (cis) or on the opposite side (trans). If the bonds which hold the substituents are both pointing upwards or downwards, then we can conclude that the compound is cis. However, if one of the bonds that holds the substituent points up and the other down, or vice versa, then the compound is trans. Looking at all four compounds, it is clear that C and D are cis isomers because in C, both the substituents point downwards, and in D, both substituents point upwards. If the question had asked for trans isomers, then A and B would be correct for trans isomers because the substituents point in directions.
Bruice,Paula Y. “An Introduction to Organic Compounds.” Organic Chemistry.Ed.Nicole Folchetti.Boston:Prentice Hall,2011. 115