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consider the hydrocarbon below.
-first image-
which of these is a trans- isomer of the above hydrocarbon?

options;

The correct answer is structure 4.

Explanation:

Isomers are the compounds which have same molecular formula but different structural formula. Cis and Trans isomers are the geometrical isomers.

Cis isomer is the one, where the groups having highest priority are on the same side of an unsaturated compound.

Trans isomer is the one, where the groups having highest priority are on the opposite side of an unsaturated compound.

For the given compound,

The highest priority group on one side of the alkene is methyl group and on the other side of the alkene is ethyl group. The given compound is a cis-isomer.

So, the trans isomer of the given compound will be that, where, methyl and ethyl group on the opposite side, which is represented by Structure 4.