Step 1 - Remembering cis vs trans compounds
Whenever a molecule has a double-bond, it allows for cis/trans geometry. We will only see such isomers when the groups bonded to the double bond are different.
We call it "cis" when two identical groups (or similar ones) belong to the same "side" of the molecule. We call it "trans" otherwise. Let's look at an example:
Step 2 - Brief revision on naming organic molecules
a) Count the number of C atoms: this will give you the prefix (met, et, prop, but, pent, ..)
b) Look for double or triple bonds between C atoms: this will give you the infix (-an-, -en-, -in-). Don't forget to specify to which C the double bond is firstly attached.
c) Look for any functional groups or geometrical features (such as cis/trans isomery)
Step 3 - Naming the compound
Let's rewrite the formula of the compound as:
We can see the H are both in the same side of the molecule (bellow the red line). Therefore, this is a cis isomer. We can already exclude all alternatives containing "trans".
Note there are 6 C atoms, which means the prefix must be "hex". The double bond starts in the 2nd C (yellow numbers in the drawing). Therefore, its name will be cis-4-hexene.