doubt

Be^- is the least stable because it has one electron (unpaired ) in the p-orbital
if you look at the other configurations
Li^-= 1s²2s²(in it all subshells are saturated ...so it is stable)
Be^- =1s²2s²2p¹(very much less stable)
B^- =1s²2s²2p² (less stable)
C^- = 1s²2s²2p³ (stable due to half filled p-orbital)

i think it should be boron anion
becoz less than half filled orbitals are least stable....

get it confirmed from aieeee!!!!!

I think its B^-.This is because Be has an exception of high e^- affinity than B atom,and Be^- is formed with a release of very high amt. of energy, making it a bit thermally unstable.
it gets converted into its ground state,as soon as its formed.

and guys, i hv had both the explanation provided by u, in 2 different sources.so,the measurement of stability using the no. of e^-s just more or less than half filled or fullfilled orbitals is controversial.better don't use it.

this question was once given in the JEE....i have clarified form many solution books and also from my chemistry teacher...really...my answer cannot be wrong !!!!

got a point in support of Be^-.

actually,Be has a very stable configuration,due to its full-filled shells.thus,forming Be^- means forming an ion with unstable configuration,from a stable element;this involves a lot of energy and thus Be^- remains as an excited ion.
while B has an unstable configuration,turning into another unstable ion B^-.

so,its difficult to say now.but sources say it to be Be^-.