Final answer:
The box that contains the next clue is the box labeled 'Present' since the equation on top of that box has exactly one solution.
Step-by-step explanation:
The question asks about the equation on top of the box that has exactly one solution. When an equation has exactly one solution, it means that the equation represents a linear equation with one unique point of intersection between two lines. In other words, the two lines represented by the equation intersect at one and only one point. Therefore, the box that contains the next clue is the box labeled 'Present'.