What may cause broccoli in a vegetable mixture to turn olive green during holding?

The correct answer is that broccoli in a vegetable mixture may turn olive green due to the presence of baking soda. When baking soda is added to vegetables during cooking, it increases the pH level, leading to a reaction that can alter the chlorophyll - the pigment that gives broccoli its vibrant green color. Under alkaline conditions, chlorophyll can break down and turn into a dull olive green color, which is often undesirable in terms of appearance.

Other substances like lemon juice, vinegar, and salt have different effects. Lemon juice and vinegar are acidic, which helps to preserve the bright green color of vegetables. Salt, while it can impact the texture and flavor of vegetables, does not significantly change their color in the same way that baking soda does when it raises the pH level.