For the structural formula,

What is the correct IUPAC name :
\[1)\]\[4 - Isopropyl - 2 - hexyne\]
\[2)\]\[4 - Ethyl - 5 - methyl - 2 - hexyne\]
\[3)\]\[3 - Isopropyl - 4 - hexyne\]
\[4)\]\[2 - methyl - 3 - ethyl - 4 - hexyne\]

Hint: We need to know that the full form of IUPAC is the International Union of Pure and Applied Chemistry. It is a World Wide organisation for the nomenclature of the new elements in the periodic table and chemical molecules. The IUPAC has certain rules and regulations for the naming of the organic compound. In that method only we named the organic molecules in World level. The naming of any organic molecule depends on the parent saturated hydrocarbon.

Complete answer:
We can draw the given structure as,
E.
The rules for naming structure is,
First to calculate the number of carbon atoms in the given ring structure. According to that count we consider our parent hydrocarbon. This count is dependent on the chain having triple bond in the molecules.

There are six carbon atoms in the chain containing triple bonds in the molecule. The number of the chain is also dependent on the carbon having should come in minimum number. Hence, we are given from right to left side of the molecule.
Hence the parent name of the molecule is hexyne. Because six carbon atoms in the chain contain triple bonds in the molecules. In this structure the second position has a triple bond in the molecule.
Ethyl and methyl are the substituents in the given structure. In fifth carbon contain methyl group and fourth carbon contain ethyl group.
Hence, the IUPAC name of the given structure is \[4 - Ethyl - 5 - methyl - 2 - hexyne\].
According to the above discussion, we conclude the IUPAC name of the given structure is\[4 - Ethyl - 5 - methyl - 2 - hexyne\].

Hence, the option \[4\] is the correct answer.

Note:
We need to know that the functional group of the molecule is very important for the naming time. If one molecule having more than one functional group means some priority list is also provided from IUPAC. If the number of substituent and functional group is dependent on the minimum number will come from the left side or right of the terminal carbon atom in the chain. If it is a cyclic structure it depends on the number we choose clockwise numbering or anti-clockwise numbering in the structure. The main chain is dependent on the large number carbon atom chain in the molecule.