Which instrument is used to trim the impression?

The correct choice for trimming an impression is the lab knife. This instrument is specifically designed for precise cutting and trimming tasks in the dental laboratory, particularly for modifying impressions and model materials. Its sharp blade allows the dental professional to achieve clean edges and accurate shapes when preparing the impression for further processing, such as pouring up models or creating dental appliances.

The other instruments listed serve different purposes. The plaster spatula is typically used for mixing plaster or other dental materials but does not have the cutting edge needed for trimming. The model trimmer is used to shape and smooth dental models, but it is not for direct trimming of impressions. The extruder gun is utilized to dispense materials like impression compounds or dental wax but does not play a role in trimming impressions either. Knowing the specific functions of these instruments helps in identifying which tools are best suited for each task in dental practice.