Which of the following represents a cis isomer?

Isomers are chemical compounds with the same molecular formula but with a different arrangement of atoms.

In Cis–isomer, both the substituents point in one direction. If one is equatorial then the other needs to be axial or vice-versa.

In Trans-isomer, both the substituents point in the opposite direction. Either both are equatorial or both are at axial position.

A. One methyl group points upward and one downward, therefore it is a trans isomer. Both are at the axial position.

B. Here also one methyl group points upward and one downward, therefore it is trans isomer. Both are in the equatorial position.

C. Here both the methyl groups point downwards, so it is a cis isomer. One is equatorial and the other axial.

D. Here both the methyl groups point upwards, so it is cis isomer. One is equatorial and the other axial.