What alkene would give only a ketone with three carbons as a product of oxidative cleavage?
The only possible alkene is 2,3-dimethylbut-2-ene.
Oxidative cleavage of an alkene is the conversion of the alkene carbons into separate carbonyl groups.
We can work backwards from the products and figure out what the alkene must have been.
The only three-carbon ketone is acetone,
When we work backwards, we find that the starting material must have been 2,3-dimethylbut-2-ene,