In order to find the margin of error, we need to know that the width of the confidence interval is equal to 2 times the margin of error.
The width of this confidence interval is given by:
[tex]\begin{gathered} width=upper\text{ }limit-lower\text{ }limit\\ \\ width=78-72\\ \\ width=6 \end{gathered}[/tex]
If the width is 2 times the margin of error, we have:
[tex]\begin{gathered} width=2\cdot E\\ \\ 6=2\cdot E\\ \\ E=\frac{6}{2}\\ \\ E=3 \end{gathered}[/tex]
The margin of error is equal to 3, therefore the correct option is D.
