Answer
See below.
Work Step by Step
$\hat{p}=\frac{x}{n}=\frac{80}{80}=1$
The z-value belonging to the $95\%$ confidence interval according to the table is $z=1.96$, thus the confidence interval is: $\hat{p}\pm z\sqrt{\frac{p(1-p)}{n}}$, which here is: $1\pm 1.96\sqrt{\frac{1\cdot(1-1)}{80}}$, thus the confidence interval is $[1,1]$, thus the answer is (a).