Doubt from isomerism

moderate
isomerism
chain-isomerism
finding-isomerism
unsolved
pp
#1

#2

According to me,these are chain isomers.
@sai_2020_2

#3

Reason??

#4

There is rule called isomeric no. For chain isomer isomeric number should be different and for position isomer isomeric number should be same.