The independent quantity is the time. We can express the total numbers of words read by a period of time using the rate at which the person reads multiplied by the total time reading.
The independent quantity is the minutes.
Therefore, the option is minutes per word, because the independent quantity is the minutes:
[tex]\frac{W}{m}\cdot t[/tex]We have that W/m is the rate, and t is the time. For example, a person that reads 90 words a minute, can read 180 words in two minutes:
[tex]90\cdot2=180[/tex]