At some high temperature the value of Kw is equal to 10^–12M². Then the H⁺ ion concentration of water at that temperature is

 We know that when temperature of water changes its ionic product also change.

$K_w=\left[H^{+}\right]\left[O{H}^{-}\right]$

water is always neutral i.e., hydrogen ion concentration is equal to hydroxide ion concentration.

$$\begin{gathered} \left[{10}^{-12}\right]={\left[H^{+}\right]}^2 \\ \left[H^{+}\right]={10}^{-6} \end{gathered}$$

Hence concentration of hydrogen ion concentration of water will be 10^-6

So, the correct answer is option (B).