Answer
(b)
Work Step by Step
By using the chain rule, we have
$$
\frac{\partial f}{\partial r}=\frac{\partial f}{\partial x}\frac{\partial x}{\partial r}+\frac{\partial f}{\partial y}\frac{\partial y}{\partial r}.
$$
See the summary on page 807 of the book.
Thus, the correct answer is (b).
