Which clause refers to individuals "held to service or labor" in the Constitution?

The reference to individuals "held to service or labor" in the Constitution pertains to the 3/5 Clause, which is found in Article I, Section 2. This clause was part of the compromise between northern and southern states during the drafting of the Constitution. It determined how enslaved individuals would be counted for purposes of representation in Congress and taxation. Specifically, it stipulated that three-fifths of the enslaved population would be included in a state's population count when determining representation and taxation. This clause reflects the contentious nature of slavery in the United States and the political compromises made during the formation of the government.

The other choices refer to different aspects of law and governance: the Equal Protection Clause addresses civil rights and the equitable treatment of individuals under the law; the Full Faith and Credit Clause ensures that states honor the public acts and judicial proceedings of other states; the Commerce Clause grants Congress the power to regulate commerce among the states. Each of these plays a vital role in the structure and function of government, but only the 3/5 Clause directly addresses the issue of service or labor within the context of enslaved individuals.