What is the solution to the equation x² = 16?

To solve the equation x² = 16, we begin by taking the square root of both sides. The square root of 16 is 4; however, it is important to remember that when solving an equation involving a square, there are typically two solutions: a positive solution and a negative solution. This is because both 4² and (-4)² equal 16.

Thus, we derive that x can be either 4 or -4. Therefore, the complete solution to the equation is x = 4 or x = -4. This encompasses both possible values that satisfy the original equation, confirming that D is indeed the correct answer. This understanding underlines the fundamental principle that equations involving squares have both positive and negative roots.