To determine the correct answer to the provided question, we need to evaluate the scenario where a research scholar did not acknowledge the help and support of respondents to questionnaires in their thesis preface.

In the field of research, ethical behavior includes acknowledging contributions from others, proper citation of sources, and honesty in research practices. Failing to acknowledge the contribution of respondents can be seen as disrespectful to their time and effort, and it violates ethical norms of research.

1. Understanding Ethical Practices:
   • Ethics in research requires that all individuals or entities that contributed significantly to the research are properly acknowledged.
   • Acknowledgment is a fundamental aspect of giving credit to those who have contributed, directly or indirectly, to the creation of the content.
2. Evaluating the Given Options:
   • Technical lapse: This refers to errors related to the technical aspects of the work, such as formatting or methodological errors, but does not relate to ethical considerations.
   • Wilful negligence: This implies knowingly ignoring a duty or responsibility. While it could relate to this context, it is more about intention rather than ethics per se.
   • Unethical act: This refers to actions that violate accepted professional ethics or moral standards. In this case, not acknowledging help is indeed an unethical behavior.
   • Lack of respect: While this indicates a disrespectful act, in the context of research it more specifically concerns ethical conduct.
3. Conclusion:
   • The failure to acknowledge respondents is an unethical act because it fails to meet the ethical obligation of recognizing the contributions of others in the research process.

Based on the above evaluation, the correct answer is that the scenario describes an unethical act.