Answer:
- 4-methylcyclohexene (the last choice)
Explanation:
The compound is an alkene (cycloalkene), with a methyl group as substituent (it substitutes one hydrogen in the carbon chain).
The IUPAC's rules state that the location of the carbon-carbon double bond in the structure is indicated by specifying the number of the carbon atom at which the C=C bond starts, assigning the lowest possible number to the double bond: this in this case is number 1.
Cyclohexene is the main chain and mehtyl is a substituent, as already said.
So, the name 1-methyl-3-cychlohexene is not valid (position 1 is for the carbon-carbon double bond).
The name methycyclohexene is not valid because it is not telling the position of methylgroup.
The name 5-methylcyclohexene is not valid because the position five should be named 2 in the cyclohexene (you must use the smallest number), so the name should be 2-methyl... instead of 5 methyl...
1-methyl-4cyclohexene is not valid because, as said, the position 1 is reserved for the carbon-carbon double bond.
Only 4-methylcyclohexene is a valid name.
The file attached shows the structure. I have added numbers on the carbons of the main chain to show you how that the methyl group is in the positiion number 4.
