To begin with, you know that five squares can be seen when a person looks down on the figure. Each stack must be at least one square tall. Therefore, you can conclude that five of the eleven cubes must be on the lowest level of the figure. No additional information is given about the heights of any of the five stacks that make up the figure. The remaining six cubes could be spread out over the five base cubes, or they could all be piled in one stack. The question asks for the greatest number of cubes that could be in one stack, therefore, pile the six remaining cubes on top of one of the base cubes. Six cubes plus the base cube equals seven cubes.
The correct answer is d) 7.