C. Words per minute, because the independent quantity is the minutes.
Step-by-step explanation:
The speed will be measured in words per minute as the number of words a person can read in a minute are counted.
The independent quantities are those quantities which are controlled during the measurement while the dependent quantities change depending on the independent quantities. While measuring the reading speed of a person minute is controlled while the number of words are counted in a minute. So minute is the independent quantity.
