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A. Incorrect. Make sure that you include x in the average of the exam scores. You may have only summed the first four exam scores when calculating the exam average: $\frac{100+60+80+30}{5}=\frac{275}{5}=54$.

B. Incorrect. Make sure that you include the fifth exam in the average. You may have only averaged the first four exams: $\frac{100+60+80+30}{4}=\frac{270}{4}=67.5$.

C. Incorrect. Keep in mind that the average should be set equal to 72. You may have included 72 in the exam average: $\frac{100+60+80+30+72}{5}=\frac{342}{5}=68.4$.

D. Correct. To calculate the mean (average) score of the 5 tests, sum the individual scores and divide that sum by the number of tests: $\frac{100+60+80+30+x}{5}=\frac{270+x}{5}$. Set this fraction equal to the given average score, $\frac{270+x}{5}=72$; then, multiply by 5 and subtract 270 to calculate $x=90$.