Correct Answer: D

Rationale: Mr. Takashi should be allowed to participate in the study only if it is likely to benefit him directly. This decision aligns with the principle of beneficence in research ethics, which emphasizes maximizing benefits and minimizing risks for research participants. Since Mr. Takashi is incapable of giving consent, his participation should be based on the likelihood of direct benefits to him rather than solely for the benefit of others or solely for research purposes. Option A is too restrictive and does not consider potential benefits to Mr. Takashi. Option B is flawed as it disregards the need for the study to benefit Mr. Takashi. Option C focuses on benefiting others, which is not the primary consideration when the participant cannot give consent.

Correct Answer: D

Rationale: Cross-sectional studies can be threatened by cohort effects because different age groups may have experienced different historical or social contexts, influencing the results. This disparity can lead to invalid comparisons between age groups, confounding the study's findings. Option A, practice effects, refer to changes in participants' behavior due to repeated testing, not a specific threat to cross-sectional studies. Participant dropout (Option B) is a threat related to attrition in longitudinal studies, not cross-sectional studies. Sequential timing (Option C) does not pose a threat to cross-sectional studies; instead, it refers to the order of tasks in a study design.

Correct Answer: B

Rationale: One challenge associated with longitudinal research is that participants may relocate or drop out of the study over time. This can result in missing data and potential biases in the findings, impacting the study's overall validity and reliability. Furthermore, it can present difficulties for researchers in accurately monitoring and analyzing changes in individuals over an extended period. Options A, C, and D are incorrect because longitudinal research can indeed allow for inferences about cause-and-effect relationships, findings can be generalized to the real world with appropriate design and sampling, and it is suitable for studying individual developmental patterns.

Correct Answer: D

Rationale: Mr. Takashi should be allowed to participate in the study only if it is likely to benefit him directly. This decision aligns with the principle of beneficence in research ethics, which emphasizes maximizing benefits and minimizing risks for research participants. Since Mr. Takashi is incapable of giving consent, his participation should be based on the likelihood of direct benefits to him rather than solely for the benefit of others or solely for research purposes. Option A is too restrictive and does not consider potential benefits to Mr. Takashi. Option B is flawed as it disregards the need for the study to benefit Mr. Takashi. Option C focuses on benefiting others, which is not the primary consideration when the participant cannot give consent.

Correct Answer: D

Rationale: In this scenario, the researchers randomly assigned adolescents to either a single-grade classroom or a mixed-age classroom, which is an example of a field experiment. A field experiment involves manipulating independent variables in a natural setting to observe the effects on participants. This design allows for more control over variables compared to naturalistic observation, case studies, or natural experiments. Naturalistic observation involves observing subjects in their natural environment without manipulating any variables. A case study focuses on in-depth analysis of a single individual or a small group. A natural experiment involves observing naturally occurring events without intervention or manipulation by the researcher.