This problem involves Euler's Homogeneous Function Theorem, which relates a homogeneous function to its partial derivatives.
If $f(x, y)$ is a continuously differentiable homogeneous function of degree $n$, then it satisfies the following identity:
$x \frac{\partial f(x, y)}{\partial x} + y \frac{\partial f(x, y)}{\partial y} = n f(x, y)$
The question states that $f(x, y)$ is a homogeneous function of degree 4. Thus, we have $n = 4$. Substituting $n=4$ into Euler's theorem gives:
$x \frac{\partial f(x, y)}{\partial x} + y \frac{\partial f(x, y)}{\partial y} = 4f(x, y)$
This identity must necessarily be true for the given function. Comparing this result with the options provided, it matches Option 3.
Given $f(x, y) = x^2 - 2xy + y^2$
The complete contour of the equation $f(x, y) = 1$ is described by the option(s) ___.