Concept:For alkenes, select the longest carbon chain containing the double bond, then number the chain to give the double bond the lowest possible locant.
Explanation:The structure can be expanded as
CH3CH2C(CH3)=C(CH3)2.
The longest continuous chain that contains both doubly bonded carbon atoms has five carbon atoms, so the parent name is pentene.
Number the chain from the end nearer the double bond.
Numbering from the right-hand end places the double bond between carbon 2 and carbon 3, giving the locant 2.
If numbered from the left-hand end, the double bond would be at position 3, which is not preferred.
Thus, the parent hydrocarbon is pent-2-ene.
There are two methyl substituents: one methyl group on carbon 2 and one methyl group on carbon 3.
Using the prefix "dimethyl", the substituents are written as 2,3-dimethyl.
Combining the substituents with the parent chain gives the complete IUPAC name as 2,3-dimethylpent-2-ene.
Answer:B. 2,3-dimethylpent-2-ene