Correct Answer: valetudinarian

Solution : The correct solution is the fourth option.

Explanation : A valetudinarian is someone who is excessively anxious about their health, often imagining ailments or exaggerating minor health issues. This term precisely captures the idea of undue anxiety about one's health.

The meanings of the other options are as follows:

• Disciplinarian: This term refers to a person who enforces rules and discipline, typically in a strict manner.
• Antiquarian: This term refers to a person interested in or studying antiquities, particularly ancient artefacts or historical relics.
• Egalitarian: This term refers to a person who advocates for or believes in the equality of all people, especially in terms of social, political, and economic rights.

Therefore, the correct one-word substitute is valetudinarian .