Final answer:
The probability that a student at Warren Middle School does not have a pet is 67%, calculated by subtracting the percentage of students with pets (33%) from 100%.
Step-by-step explanation:
If 33% of the students at Warren Middle School have a pet, then the probability of the students who do not have a pet is the remainder when we subtract the percentage of pet owners from 100%, because the total probability is always 100%. So, we calculate 100% - 33% to find the percentage of students who do not have a pet. This equals 67%. Therefore, the correct answer is b) 67%.