Final answer:
The gingival sulcus depth in a dog should be assessed with a periodontal probe during a dental prophylaxis, as it is specifically meant for this measurement and critical for detecting periodontal disease.
Step-by-step explanation:
During a dental prophylaxis in a dog, assessment of the gingival sulcus depth is best performed with C. a periodontal probe. This instrument is specifically designed to measure the depth of the sulcus (the space between the gum and the tooth) and check for any signs of periodontal disease. Accurate measurement of sulcus depth is essential for identifying conditions like gingivitis and periodontitis, which if left untreated, can lead to tooth loss, bone erosion, and other serious health issues in dogs.
Dental radiography is crucial too, especially for detecting deeper decay and damage that is not visible externally, but it is not the correct tool for measuring the sulcus depth. The use of an ultrasonic scaler tip or the point of a curette in this context would be inappropriate, as they are used for removing tartar and cleaning but not for measuring sulcus depth.