I am unsure how to solve this problem and what formula

SOLUTION:
The z-score is gotten as;
[tex]Z=\frac{x-\mu}{\frac{\sigma}{\sqrt{n}}}[/tex]Inserting, the values we have;
[tex]Z=\frac{7-8}{\frac{3}{\sqrt{25}}}=-1.67[/tex]Thus, using a significance level of 0.05;
Thus, the answer is OPTION C