Step 1: Understanding the research designs.
- Cross-sectional design (C) involves studying participants from different age groups at the same point in time to compare different age-related variables.
- Longitudinal design (D) involves studying the same participants over an extended period of time to observe changes.
- Cross-sequential design (A) is a combination of both cross-sectional and longitudinal methods, studying multiple age groups over time.
- Functional design (B) is not a standard term in research design.
Step 2: Analyzing the options.
- (A) Incorrect, as cross-sequential design involves both cross-sectional and longitudinal elements.
- (B) Incorrect, functional design is not a recognized type of research design.
- (C) Correct, cross-sectional design is used to study multiple age groups at a specific point in time.
- (D) Incorrect, as longitudinal design follows the same group over time rather than different age groups at one point.
Step 3: Conclusion.
Thus, the correct answer is (C) Cross-sectional design.