Which of these functions is homogeneous of degree 2?

Explanation

1. 01

   Replace $x, y$ by $\lambda x, \lambda y$; a homogeneous function gives back $\lambda^{n}$ times itself.

2. 02

   $x^{2} - 3 x y + y^{2}$ gives $\lambda^{2}$ times itself; each of the others fails.