What must be done to the plastic film before using the tray?

The correct answer is that the plastic film must be pierced before using the tray. This step is often necessary when cooking or heating food in a container covered with plastic film, as it allows steam to escape. If the film is not pierced, steam can build up underneath it, potentially causing the film to bubble or even burst. This could lead to uneven cooking, splattering, or an inefficient cooking process.

Removing the film completely would not be ideal if the intention is to keep the moisture in or to allow food to steam, while folding or heating the film does not address the need for steam release, which is crucial for safe and effective cooking. Therefore, piercing the film is the most practical and effective option to ensure proper heat circulation and safety when using the tray.