If the points (0, 0), (1, 2) and (x, y) are collinear, then :

1. x = y

2. 2y = x

3. 2x = y

4. y = -x

Since, points (0, 0), (1, 2) and (x, y) are collinear.

∴ Slope of line joining points (0, 0) and (1, 2) = Slope of line joining (1, 2) and (x, y).

$\Rightarrow \dfrac{2 - 0}{1 - 0} = \dfrac{y - 2}{x - 1} \\[1em] \Rightarrow \dfrac{2}{1} = \dfrac{y - 2}{x - 1} \\[1em] \Rightarrow 2(x - 1) = 1(y - 2) \\[1em] \Rightarrow 2x - 2 = y - 2 \\[1em] \Rightarrow 2x = y - 2 + 2 \\[1em] \Rightarrow 2x = y.$

Hence, Option 3 is the correct option.