Why does adding white not necessarily change the temperature of a cool colour?

Correct answer: White mainly increases the colour's value (lightness)

The governing concept is a tint: a hue mixed with white. White mainly raises value, making the colour lighter, and it usually lowers saturation. Temperature, however, depends chiefly on the hue’s directional bias, such as blue-green or blue-violet. Adding white alone does not automatically move that hue into the warm range. It cannot create a complementary colour, and it generally does not increase saturation. Thus, option A is correct.