Which of the following products is used to alter the structure of the hair temporarily?

The product that alters the structure of the hair temporarily is gel. Hair gel is designed to provide hold and texture to hairstyles by coating the hair strands and allowing them to maintain a specific shape and style for an extended period while also being water-soluble. This means it can be easily washed out, returning the hair to its natural state.

On the other hand, perm solutions chemically restructure the hair’s bonds to create curls or waves, which is not a temporary change. Hair mousse helps to add volume and define curls but does not alter the hair structure. Heat protectants are used to shield the hair from damage during styling with heat but do not change the hair's structural composition. Thus, gel is the product that allows for temporary styling alterations while remaining easy to remove.