Answer:
A. A double bond
Step-by-step explanation:
Trans and cis Isomerism is a form of isomerism that specifically refers to alkenes, which is to say, aliphatic chains containing a double bond between two carbon atoms. Technically speaking, options B. and C. are both correct, because an alkene contains aliphatic hydrocarbons. However, neither of those options necessarily relate to cis and trans isomers.