(d)
$|\overrightarrow{\mathbf{a}} - \overrightarrow{\mathbf{b}}|^{2} = |\overrightarrow{\mathbf{a}}|^{2} + |\overrightarrow{\mathbf{b}}|^{2} - 2|\overrightarrow{\mathbf{a}}||\overrightarrow{\mathbf{b}}|\cos\theta$
$= 1^{2} + 1^{2}2 \bullet 1 \bullet 1 \bullet \cos{60{^\circ}}\ \ \ \ \ \ \ \lbrack\because\left| \overrightarrow{\mathbf{a}} \right| = |\overrightarrow{\mathbf{b}}| = 1\rbrack$
$= 2 - 2 \bullet \frac{1}{2} = 1$