A block of iron sinks in fluid A but floats in another liquid B, this is because:
Density of fluid A < Density of fluid B
Since we know that the sinking and floating of a body in a liquid is due to the difference in their densities. If the density of the body is greater than the liquid then it sinks and vice-versa. So, the density of fluid B must be greater than iron and thus also greater than fluid A.