Answer:
d. Cross-sectional
Step-by-step explanation:
In research, a cross-sectional study refers to a type of study where the researcher makes observation on a sample at a specific point of time. In other words, the researcher wants to know what is happening in a group at a specific point of time and the data is gathered at one specific point of time too.
In this example, Dr. McLean wants to study the cognitive skills of people in their 30s, 50s, and 70s and he collects the data over the course of a day. We can see that the researcher wants to know what is happening at a specific point of the lifespan (30s, 50s and 70s) and that he/she is collecting the data at one specific point of time (one day). Therefore, this would be a cross-sectional study.