Final answer:
Disintegrations per minute is a measure of radioactivity used to calculate the age of a sample.
Step-by-step explanation:
The activity of a sample of a radioactive substance is usually measured in disintegrations per minute (dpm). This measurement is used to calculate the age of the sample.
Radioactivity is determined by measuring the number of decay processes per unit time. Each count measures a single decay process. The higher the activity (number of counts/minute), the more radioisotope is present in the sample.
Therefore, the correct option in this context is A. Measure of radioactivity.