This is the best answer. Recall the probability that we obtain a sample mean of 580 or larger in a random sample of n = 10 from a population with a mean of 555 is .2843. This means that 28% of the time that we take a sample of 10 people from a population with a mean of 555, the sample mean will be 580 or above.
The most important issue to understand here is that our sample mean of 580 is not surprising given an assumption that the population mean is 555. Since the sample mean is likely (i.e., not surprising), we cannot claim that the program graduates score better than 555 on average. Statistically speaking we would say "we cannot reject the null hypothesis that the population mean for this GRE training program is 555."
Now, you will go through several interactive examples that examine hypothesis testing. Go to interactive examples