Kara wants to make a histogram of the number of students who wear blue shirts in her class throughout the week. She gathered the following information:

Day 1 4 students

Day 2 9 students

Day 3 5 students

Day 4 7 students

Day 5 6 students

What data would be on the horizontal axis?
names of the students wearing the blue shirts
the exact color of the blue shirt
school day
number of students wearing blue shirts