Introducing Statistics: Do Those Points Align Practice Test

After collecting data on the number of years a person has been running and their time to complete a 10k race, a coach finds a negative linear association in her sample. Which question is most directly related to the statistical concept of inference for regression slope?

Imagine a population where for any given value of an explanatory variable $$x$$, there is a distribution of corresponding values for a response variable $$y$$. If a linear relationship exists, the means of these distributions fall on a straight line. Why would individual points from this population not fall perfectly on that line?