Slope is $\frac{\text{change in }y}{\text{change in }x}$.

As $x$ increases, see whether the line goes up or down to decide if the slope is positive or negative.

Enter the points from the graph, such as $A=(0,3)$ and $B=(2,1)$.

In a new line, type (y(B)-y(A))/(x(B)-x(A)) and evaluate the result.

The computed value is $-1$, so the correct choice is $-1$.

From the graph, the line passes through the points $(0,3)$ and $(2,1)$.

Therefore, the slope of the line is $-1$.