You run an experiment to test whether consumers are more likely to purchase a product if its orange packaging is more intense in colour. Which of the following would be an essential feature of the experiment for you to be able to conclude that the packaging with the most intense orange colour is liked best? $($In other words, without this feature, we simply cannot consider the effort to be a true experiment.$)$

Question $7$ options:

You observe a correlation between the intensity of the orange colour and different shapes of product packaging.

Using realistic experimental stimuli $($for example, product packaging is already familiar to participants$).$

Randomly assigning consumers to different conditions with orange product packaging that vary in the intensity of colour.

None of the above.