Answer to the Question: Does the fundamental have the highest or lowest fequency?

You answered that the fundamental has the lowest frequency.

Examine the figure below which shows both the fundamental frequency and the 25th harmonic. The lower the frequency, the fewer cycles will occur. So, in this figure, the 25th harmonic has the highest frequency and the fundamental has the lowest frequency for the square-wave grating. In fact, it is most common for the fundamental to be the lowest frequency.

Thus your answer is CORRECT.