Final answer:
The correct answer is C. an average of 18 percent of the television households in the country were tuned to Grey's Anatomy.
Step-by-step explanation:
The national program rating is a measure of the percentage of television households that watched a particular program. In this case, the average national program rating of 18 means that, on average, 18 percent of the television households in the country were tuned to Grey's Anatomy during the 2007 season.
For example, if there were 100 television households in the country, the average rating of 18 would mean that 18 of those households were watching Grey's Anatomy. This rating provides information about the popularity and viewership of the show.