Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 121 / 1 / i / d / Solution

U+D. The natural numbers are absolute between transitive models of ZFC, and the statement $x\subseteq \mathbb{N}$ is the bounded assertion $\forall y\in x(y\in \mathbb{N})$. Hence the property of being a subset of $\mathbb{N}$ is absolute.