In a one-sample $t$-test, what does the alternative hypothesis state regarding the population mean ($\mu$) and the hypothetical comparison mean ($\mu_0$)?

A researcher uses a one-sample t-test to determine whether the average anxiety score of her sample differs from a known population average of 50. In this test, the null hypothesis predicts that any difference between her sample mean and the value of 50 is due to sampling error alone, rather than reflecting a true difference in the population mean.