Consider the following irrotational $3$D flow: $V=4$axi$-4$ayj. Knowing that a is a constant, Derive the expression of the velocity potential $\frac{O}{x,y}?$

a$.\frac{O}{x,y}=ax-ay+c$

b$.\frac{O}{x,y}=2a{x}^2-2a{y}^2+c$

c$.\frac{O}{x,y}=2ax-2ay+c$

d$.\frac{O}{x,y}=a{x}^2-a{y}^2+c$