In this, there are possibilities of selecting many 8 carbon chains but the choice that gives the simplest substituents is chosen and that’s why the highlighted chain is chosen.

If we started numbering from left side then methyl group will come on 7^th C and the 2 ethyl groups will be on 3^rd and 4^th C and if we started numbering from right side then methyl group will come on 2^nd C and the 2 ethyl groups will be on 3^rd and 4^th C. Thus, numbering is done from left side so that all the substituents get the lowest possible number.

There are 2 ethyl and 1 methyl group present on C-3, 6 and 2 respectively. And ethyl comes before methyl alphabetically.

Hence, the correct IUPAC name is (i) 3, 6 – Diethyl – 2 – methyloctane.