Which one of the following is an example of panel data set?

a. The number of firm establishments for all states in US in 2016
b. A health survey looks at 200 people's medical spending from 1992 to 2012.
c. The 2010 U.S. Census, which surveyed various aspects of the U.S. population.
d. Daily gas prices at your local gas station over the course of a year