Answer: Option B
:
B
Here we are going to compare these with each other and find out which is the least stable. But these species might not exist due to inherent instability.
Electronic configuration of $L{i}^{-}$ - ($2$) fully-filled, so relatively more stable.
$B{e}^{-}$ - ($2,1$) less stablethan $Be$ ($2$);
$B^{-}$ - ($2,2$) relatively stable;
$C^{-}$ - ($2,3$) relatively stable.