The major isolated organic product of the reaction shown is (a) octane. Therefore , (a) octane is correct.
The reaction shown is a catalytic hydrogenation reaction, which adds hydrogen gas to a carbon-carbon double bond.
In this case, the carbon-carbon double bond is in the middle of the alkene chain, so the reaction will produce an alkane with no multiple bonds.
The other choices are all possible products of the reaction, but they are less likely to be formed in high yield.
Cis- and trans-4-octene are both alkenes with a carbon-carbon double bond at the fourth carbon atom.
These products can be formed if the hydrogen gas is added to the alkene in a specific way, but it is less likely than the formation of octane.
The addition of hydrogen gas to the carbon-carbon double bond in the octane molecule does not change the carbon skeleton of the molecule. Therefore, the major isolated organic product of the reaction is still octane.