(d)
$\left| \overrightarrow{\mathbf{a}}\mathbf{-}\overrightarrow{\mathbf{b}} \right|^{2} = \left| \overrightarrow{\mathbf{a}}\left. \ \right|^{2} + \right|\overrightarrow{\mathbf{b}}\left. \ \right|^{2} - 2\ \left| \overrightarrow{\mathbf{a}} \right|\left| \overrightarrow{\mathbf{b}} \right|\cos\theta$
$\Rightarrow \ \left| \overrightarrow{\mathbf{a}}\mathbf{-}\overrightarrow{\mathbf{b}} \right|^{2} = 1 + 1 - 2\cos{60{^\circ} = 2 - 1}$
$\Rightarrow |\overrightarrow{\mathbf{a}}\mathbf{-}\overrightarrow{\mathbf{b}}| = 1$