Please take note that, according to the statement, the height of the box is "x-1".
That fact alone is enough to force any modeling function to only use x values larger than 1.
Similarly, the width being "5-x" forces the modeling function to use values for x that are less than 5.
This graph includes values for x that must be excluded, so it is not a good modeling function.
In addition, words such as "fastest" when used without a reference create an implication of time as a parameter. Combine the lack of a time factor in this question together with the issue of a box with a changing volume (the first sentence in the question refers to "boxes" where as the rest of the question refers to a single box), and confusion in the mind of the test taker is to be expected.
This question could have been written quite differently and perhaps have been restored.
When the day comes when these tests can be truly computer-based, a question such as this might be accompanied by a visual such as that shown here:
In the meantime perhaps New York could stop its insistence on testing questions being "real world".