Past exam of the options I course of the University of Oxford / 2011 / E / 2 / c / Solution

For the quotient map $q:X\to X/\sim$, the quotient topology is defined byTake the Hausdorff space $X=\mathbb{R}$ and identify two numbers precisely when they are both rational or both irrational. The quotient has two points. The inverse images of its singleton subsets are $\mathbb{Q}$ and $\mathbb{R}\setminus \mathbb{Q}$, neither of which is open in $\mathbb{R}$, since each is dense and has dense complement. Hence the quotient has the indiscrete topology and is not Hausdorff.