Final answer:
The detail addressing constitutional principles from the 'address to the Niagara Movement' is the call for the enforcement of the 14th and 15th Amendments, reflecting the need to protect civil liberties and voting rights for African Americans.
Step-by-step explanation:
The detail from the 'address to the Niagara Movement' that deals with the application of constitutional principles is the option stating: 'The time has come for the nation to work for the enforcement of the 14th and 15th Amendments to the Constitution of the United States'.
This statement directly references the need for active enforcement of constitutional amendments designed to protect civil liberties and the right to vote for African Americans.
The 14th Amendment includes provisions for citizenship rights and equal protection under the law, while the 15th Amendment specifically prohibits denying the right to vote based on race, color, or previous condition of servitude.
In contrast, the other options focus on broader social and economic reforms that, while important, do not specifically relate to the enforcement of constitutional amendments.