The correct IUPAC name of the compound, 

From the structure, the longest chain containing the double bond has $10$ carbon atoms, so the parent is $decene$. Numbering the chain to give the double bond the lowest locant places it at $4$, giving $dec ext{-}4 ext{-}ene$. With this numbering, the substituents are one methyl at $3$ and two ethyl groups at $5$ and $6$. Listing substituents alphabetically gives $5,6 ext{-}diethyl ext{-}3 ext{-}methyldec ext{-}4 ext{-}ene$. Comparing with the options, this matches option $C$.