The value of the ionisation constant, $K_w$, of a sample of pure water at different temperatures is shown in the graph below - VCE - SSCE Chemistry - Question 8 - 2009 - Paper 1

As the temperature of water increases, the ionisation constant $K_w$ also increases. This results in greater concentrations of hydrogen ions, $[H^+]$, and hydroxide ions, $[OH^-]$. Since pure water's neutrality is defined by the equality of $[H^+]$ and $[OH^-]$, the increase in ionisation leads to a rise in $[H^+]$. Consequently, the pH, which is given by the equation

$pH = -\log[H^+]$

will decrease as more hydrogen ions are present, suggesting the water remains neutral but with a lower pH, affirming option B.