What clause did the supreme court use to apply the bill of rights to the states during the 20th century

the due process clause

Explanation:

The Fifth and Fourteenth Amendments to the United States Constitution each contain a due process clause. Due process deals with the administration of justice and thus the due process clause acts as a safeguard from arbitrary denial of life, liberty, or property by the government outside the sanction of law.

its c

both include an amendment process.