The flux density at a point in space is given by  $\textbf{B}=4x \textbf{a}_x+2ky \textbf{a}_y+8 \textbf{a}_z \:\: Wb/m^2$. The value of constant $k$ must be equal to
1. $-2$
2. $-0.5$
3. $+0.5$
4. $+2$