in reality which is heavier

1 kg of cotton or 1 kg of iron

explain

The 100 kg iron is heavier.

Upthrust acts on both- iron and cotton. Cotton has much lower density as compared to iron. Thus, the upthrust is more on cotton as compared to iron.

We know that UPTHRUST= Actual weight-apparent weight

The actual weight of iron and cotton is 100kg.

Knowing that the upthrust is more on cotton, let us assume that the upthrust on cotton is 12N. 

Upthrust = Actual weight - Apparent weight

12 = 100 - Apparent weight

Apparent weight(cotton) = 88

We know that the upthrust on iron is less. So let us assume that the upthrust on iron is 10N.

Then, 

Upthrust= Actual weight - Apparent weight

10 = 100 - Apparent weight

Actual weight (iron) = 90

Thus, iron is heavier.