Fixed

You can either draw or visualize.

The point $(4a,b)$ will be at the same height $b$ as point $(a,b)$, and form the base of the triangle, whereas the point $(2a,2b)$ will be the apex of the triangle.

$base = 4a-a = 3a$ (subtracting the x-coordinate of the points $(a,b)$ and $(4a,b)$)

$height = 2b-b$ (subtracting the y-coordinate of $(2a,2b)$ and $(a,b)$)

$\text{ Area of Triangle } = \dfrac{1}{2} \times base \times height = \dfrac{1}{2} \times 3a \times b = \dfrac{3ab}{2}$

The answer is Choice C