A metric space is complete if every Cauchy sequence converges to a limit within the space. $\mathbb{R}$ (with usual metric) is complete — this is a fundamental property. $\mathbb{Q}$ is not complete (Cauchy sequences of rationals can converge to irrationals). $[0,1]$ is complete (closed subset of $\mathbb{R}$). $\mathbb{Z}$ with usual metric: Cauchy sequences in $\mathbb{Z}$ are eventually constant, so $\mathbb{Z}$ is complete too. $\mathbb{R}$ is the classical answer.