The correct answer is A. xdy + ydx.
The total derivative of a function of two variables is the sum of the partial derivatives of the function with respect to each variable. In this case, the function is $xy$, and the partial derivatives are $x\frac{dy}{dx}$ and $y\frac{dx}{dy}$. Therefore, the total derivative is $x\frac{dy}{dx} + y\frac{dx}{dy}$, which can also be written as $xdy + ydx$.
Option B is incorrect because it does not include the partial derivative of $y$ with respect to $x$. Option C is incorrect because it does not include the partial derivative of $x$ with respect to $y$. Option D is incorrect because it is not the total derivative of a function of two variables.