What should be mixed with dark blue to make it lighter while keeping it bright?

Correct answer: A little white

The governing colour principle is that adding white creates a tint and increases a colour’s light value. A small amount of white mixed gradually into dark blue makes it lighter while preserving its blue identity, so option B is correct. Black lowers the value and usually makes the mixture darker; brown can dull it, while extra red shifts the hue toward violet or purple.