Choice C is correct. A line of best fit is shown in the scatterplot such that as the value of $x$ increases, the value of $y$ decreases. It follows that the slope of the line of best fit shown is negative. The slope of a line in the xy-plane that passes through the points $\left(x_1,y_1\right)$ and $\left(x_2,y_2\right)$ can be calculated as $\frac{y_2-y_1}{x_2-x_1}$. The line of best fit shown passes approximately through the points $\left(0,8\right)$ and $\left(10,1\right)$. Substituting $\left(0,8\right)$ for $\left(x_1,y_1\right)$ and $\left(10,1\right)$ for $\left(x_2,y_2\right)$ in $\frac{y_2-y_1}{x_2-x_1}$ yields the slope of the line being approximately $\frac{1-8}{10-0}$, which is equivalent to $\frac{-7}{10}$, or $-0.7$. Therefore, of the given choices, $-0.7$ is the closest to the slope of this line of best fit.

Choice A is incorrect. The line of best fit shown has a negative slope, not a positive slope.

Choice B is incorrect. The line of best fit shown has a negative slope, not a positive slope.

Choice D is incorrect and may result from conceptual or calculation errors.