Final answer:
The number of inches of rainfall in a county is an example of a discrete random variable.
Step-by-step explanation:
A. The time required for a runner to finish a marathon.
B. The number of inches of rainfall in a county.
C. The temperature of a pot roast cooking in an oven.
D. The number of gallons of concrete used at a construction site.
E. The number of beverages sold at a lemonade stand.
Out of the given options, B. The number of inches of rainfall in a county is an example of a discrete random variable. This is because rainfall can be measured in specific increments (e.g., 1 inch, 2 inches, etc.), making it a countable variable. The other options involve continuous measurements or non-countable values.