Which of the following factors is crucial for protecting research participants?

Informed consent is a fundamental ethical principle in research involving human participants. It ensures that individuals are fully aware of the nature of the research, what their participation entails, and any potential risks involved before they agree to participate. This process respects the autonomy of participants by allowing them to make an informed decision about their involvement in the study. By obtaining informed consent, researchers uphold ethical standards and protect participants' rights, ensuring that they are voluntarily participating without coercion or undue influence. This protection is vital for maintaining trust between researchers and participants and is a cornerstone of ethical research practices.

The other options, while important to research methodology, do not directly address the ethical obligation to protect participants. Random sampling pertains to how participants are selected and does not guarantee their rights are upheld. Data analysis techniques and sample size relate to the validity and reliability of results rather than to the ethical considerations concerning participants' welfare.