Question

y can't the answer for 3rd one b 2 -bromo 3methyl pent-3-ene

Answer 1

There are certain rules of nomenclature which you have to abide while naming an organic compound.

1) try to give the double bond the lowest positing possible.
In this case, it is 2
2) Therefore bromine group gets the position 4. 
3) The methyl group at 3 would be considered a functional group.

hence, it would be 4-Bromo-3-methylpent(a)-2-ene