Why is adding only white insufficient to make a dark cool colour appear warm?

Correct answer: White mainly changes value, not temperature

The governing distinction is between value and temperature. Adding white creates a tint and raises the value, so a dark cool colour becomes lighter, but its hue bias remains fundamentally cool unless another colour shifts it toward yellow, orange, red, or another warmer neighbour. Option A is therefore correct. White does not make every colour blue, does not increase saturation in this situation, and is not a complementary colour, so B, C, and D are unsuitable.