Chapter 3, Problem 15PS

Which of the following is not a binary expression?

$\begin{array}{|l|}\hline \text{a}\text{. 3*5}\hfill \\ \text{b}\text{. x}+=6\hfill \\ \text{c}\text{. y}=\text{5}+\text{2}\hfill \\ \text{d}\text{. }-2\hfill \\ \text{e}\text{. y}\%\text{z}\hfill \\ \hline\end{array}$