`a)` $\overrightarrow{AB'}$ `+` $\overrightarrow{CD'}$

`=` $\overrightarrow{AB'}$ `+` $\overrightarrow{BA'}$

`=` $\overrightarrow{AA'}$ `+` $\overrightarrow{AB}$ `+` $\overrightarrow{BA}$ `+` $\overrightarrow{BB'}$

`=` `2.`$\overrightarrow{AA'}$

`=` `2.2a`

`=` `4a`

`⇒` `SAI.`

`b)` $\overrightarrow{A'D}$ `+` $\overrightarrow{CB'}$ `=` $\overrightarrow{A'D}$ `+` $\overrightarrow{DA'}$ `=` $\overrightarrow{0}$ 

`⇒` `ĐÚNG.`

`c)` `|`$\overrightarrow{AB}$ `+` $\overrightarrow{AD}$`|`

`=` `|`$\overrightarrow{AB}$ `+` $\overrightarrow{BC}$`|`

`=` `|`$\overrightarrow{AC}$`|`

`=` $\sqrt{\overrightarrow{AB}+\overrightarrow{AD}}$

`=` $\sqrt{a^2+(a\sqrt{3})^2}$

`=` `2a`

`⇒` `SAI.`

`d)` `|`$\overrightarrow{AB}$ `+` $\overrightarrow{A'D'}$ `+` $\overrightarrow{CC'}$`|`

`=` `|`$\overrightarrow{AB}$ `+` $\overrightarrow{AD}$ `+` $\overrightarrow{AA'}$`|`

`=` `|`$\overrightarrow{AC'}$`|`

`=` $\sqrt{\overrightarrow{AB}+\overrightarrow{AD}+\overrightarrow{AA'}}$

`=` $\sqrt{a^2+(a\sqrt{3})^2 +(2a^2)}$

`=` $2\sqrt{2}a$

`⇒` `ĐÚNG.`