D
There are three tests. The average is given by their sum and division by 3:
[tex]\frac{79+73+\text{??}}{3}[/tex]We want it to be 79 or more:
[tex]\frac{79+73+\text{??}}{3}\ge79[/tex]We want to find the value of the last test, ??, so the average is at least 79:
[tex]\begin{gathered} \frac{79+73+\text{??}}{3}\ge79 \\ \downarrow\text{ since 79+73=152} \\ \frac{152+\text{??}}{3}\ge79 \end{gathered}[/tex][tex]\begin{gathered} \downarrow\text{ taking 3 to the right side} \\ \frac{152+\text{??}}{3}\cdot3\ge79\cdot3 \\ 152+??\ge237 \end{gathered}[/tex][tex]\begin{gathered} \downarrow\text{ taking 152 to the right} \\ -152+152+??\ge-152+237 \\ ??\ge85 \end{gathered}[/tex]?? ≥ 85 means that the third score must be 85 or greater.
That is why the answer is D.
