Select the most appropriate one-word substitution for the given group of words. 

The therapeutic use of sunlight for curing certain disease.

Heliotherapy

The correct option is 3. Heliotherapy.

• Genealogy is the study of family trees and ancestry.
• Paleontology is the study of fossils and extinct lifeforms.
• Chronobiology is the study of biological rhythms and their relationship to time.
• Heliotherapy is the therapeutic use of sunlight for curing certain diseases.