Which method is correct for lightening blue-green while retaining its hue?

Correct answer: Add a controlled amount of white

The governing concept is making a tint. A controlled amount of white raises the value of blue-green while leaving its basic hue recognisable. The word controlled is important: excessive white can make the colour very pale and weaken its identity. Option A is correct. Red-orange or yellow-orange changes the hue through mixture, and black lowers the value and may make the colour dull rather than lightly tinted.