Explanation

Let r makes an angle $\theta$ with positive x-axis component of r along x-axis. $r_{x} = |r|\cos \theta$ $(r_{x})_{maximum} = |r|(\cos \theta)_{maximum} = |r|\cos 0^{\circ} = |r|$ ($\because \cos \theta$ is maximum of $\theta = 0^{\circ}$) $\theta = 0^{\circ}$. Hence the vector r has maximum value along positive x-axis.