Give the name and structure of the lowest molecular mass cumulative polyene that shows only geometrical isomeris `(` diastereomers `)` and not optical isomers.

Explanation

Allenes (cumulative diene) or compounds containing even number os cumulative double bonds with two different `(R^('))` groups on the terminal double bond show optical isomerism.

`(I),(II),(III),` and `(IV)` show optical isomerism.
So the simplest, with the lowest molecular mass, cumulative polyene that shows only geometrical isomers must have three cumulative double bonds with one `H` atom at terminal double bond.