What action should you take to modify the appearance of a 3D model's inside faces?

Flipping face normals is the correct approach to modify the appearance of a 3D model's inside faces. Normals are vectors that are perpendicular to the surface of a face in a 3D model and determine which direction the surface is facing. When you flip the face normals, you essentially reverse the direction they point, which can make the inside faces visible when viewed from the outside.

This is particularly important in situations where you want to render or view the internal structure of a model, such as when creating objects that are hollow or have internal details that need to be displayed. Flipping the normals ensures that the interior faces are rendered correctly from the desired viewpoint.

The other options may influence the overall appearance of a 3D model but do not specifically address the visibility or presentation of the inside faces. Changing the material affects the surface appearance but does not alter which faces are displayed. Revealing hidden faces is typically about un-hiding elements that are obstructed or removed from the scene but does not affect the direction of normals. Applying new lighting can enhance how the model looks, but it doesn't impact the fundamental way the faces are oriented or seen from different angles.