Which amendment is associated with equal protection under the law?

The 14th Amendment is closely associated with the principle of equal protection under the law. Ratified in 1868, this amendment includes the Equal Protection Clause, which states that no state shall deny to any person within its jurisdiction "the equal protection of the laws." This clause is fundamental in ensuring that individuals are treated equally by the law and serves as a critical basis for civil rights protections in the United States.

The context of the amendment stems from addressing the inequality and discrimination faced by former slaves after the Civil War. It has been used in numerous landmark Supreme Court cases to challenge various forms of discrimination, including racial and gender discrimination, thereby solidifying its importance in American legal and social frameworks.

In contrast, the other amendments listed are not directly related to equal protection. For instance, the 10th Amendment deals with states' rights, the 12th Amendment outlines the procedure for electing the President and Vice President, and the 16th Amendment relates to the federal income tax. Each of these amendments addresses distinct issues not connected to the concept of equal protection, making the 14th Amendment the clear choice for this question.