100 kg cotton will be heavier.
This is because...
Upthrust acts on both iron and cotton. Cotton has a lower density than that of iron and therefore the upthrust is more whereas upthrust is less on iron compared to cotton.
Now, we know that UPTHRUST= Actual weight-apparent weight
It is given that apparent weight is 100kg in both the causes.
We know upthrust is more on cotton.
Let us suppose upthrust on cotton is 10N
Upthrust= Actual weight- Apparent weight
10=Actaul weight - 100
110kg=Actual weight of cotton
Even if the upthrust on iron is 9.9 N
Upthrust= Actual weight - Apparent weight
9.9=Actual weight- 100
PLEASE NOT THAT WE ARE ASSUMING THE MAGNITUDE OF UPTHRUST! :)
Therefore, Cotton is heavier!