A. Sai do \(\overrightarrow {AC} - \overrightarrow {AD} = \overrightarrow {DC} \)

B. Sai do \(\overrightarrow {AC} - \overrightarrow {BD} = 2\overrightarrow {CD} \Leftrightarrow \left( {\overrightarrow {AB} + \overrightarrow {AD} } \right) - \left( {\overrightarrow {AD} - \overrightarrow {AB} } \right) = 2\overrightarrow {CD} \Leftrightarrow 2\overrightarrow {AB} = 2\overrightarrow {CD} \) (Vô lí)

C. Sai do \(\overrightarrow {AC} + \overrightarrow {BC} = \overrightarrow {AB} \Leftrightarrow \overrightarrow {AC} - \overrightarrow {AB} = - \overrightarrow {BC} \Leftrightarrow \overrightarrow {BC} = \overrightarrow {CB} \) (Vô lí)

D. Đúng do \(\overrightarrow {AC} + \overrightarrow {BD} = \overrightarrow {AB} + \overrightarrow {BC} + \overrightarrow {BC} + \overrightarrow {CD} = 2\overrightarrow {BC} + \left( {\overrightarrow {AB} + \overrightarrow {CD} } \right) = 2\overrightarrow {BC} + \overrightarrow 0 = 2\overrightarrow {BC} \).