Answer: 4. The beak size in a large number of finches was measured every year from 1981 to 1987
Explanation:
The answer in a way, is in the first sentence of the question. The researcher spent many years researching the rate of evolutionary change. This would require a lot of long term examination of a large sample of finches, across many many generations. Evolutionary change takes time and successive generations for natural selection to select for the favoured character trait. Seeing as the researcher was studying not just the occurrence of evolutionary change but it's rate, it is imperative that the a lot of observation and data collection be carried out along a long period of time. This type of study is called a longitudinal study.