The question asks to identify the developmental research design where an investigator observes children's mastery of a new task over several frequent sessions.

Understanding Microgenetic Research Design

The microgenetic design is specifically used to observe and analyze the process of change or development as it occurs. This approach involves intense study over a short, critical period, often focusing on learning or skill acquisition.

• Key Features: In a microgenetic design, researchers select a specific behavior or skill that is undergoing rapid change. They then observe participants (in this case, children) intensively over a series of closely spaced sessions. This allows for a detailed examination of the learning process, including the steps children take to achieve mastery of a novel task.
• Application to Question: This directly matches the scenario described: presenting children with a new task and following their progress and mastery through frequent, concentrated observation.

Comparing Research Designs

Let's look at why other designs aren't the best fit:

• Cross-sectional design: This method compares different age groups at a single point in time. It shows age differences but doesn't track individual change or the *process* of learning a novel task.
• Longitudinal design: This involves studying the same individuals repeatedly over a longer period. While it tracks change over time, it typically doesn't involve the extremely *closely spaced sessions* focused on the rapid mastery of a single novel task that characterizes the microgenetic approach.
• Sequential design: This design combines elements of both cross-sectional and longitudinal studies by following multiple age groups over time. While it provides rich data, the core focus isn't necessarily the intensive, short-term observation of mastering one specific, *novel task*.

Therefore, the design that focuses on detailed observation of skill development over a short, intensive period, such as learning a novel task in closely spaced sessions, is the microgenetic design.