Final answer:
The Periodontal probe is the instrument used to measure the loss of gingival attachment, which is key in assessing periodontal health.
Step-by-step explanation:
The instrument used to measure loss of gingival attachment is A) a Periodontal probe. This tool is specifically designed for this purpose, as it allows dentists and hygienists to measure the depth of the gum pockets around a tooth. When gingival attachment is lost, these pockets become deeper due to the gum tissue detaching from the tooth's surface. The periodontal probe has markings on it to indicate the depth of the pocket, effectively measuring the loss of attachment.