Basic Principles of Organic Chemistry — Chemistry STD 11 Science — Question

b. The most stable species from the given species is $CBr _3^{-}$.
This is because it contains $3- Br$ atoms, which exhibits electron withdrawing inductive effect. Carbanions are stabilized by -1 inductive (electron withdrawing) effect. Larger the number of -I groups attached to the negatively charged carbon atom, lower will be the electron density on the carbon atom and higher will be its stability.

c. The most stable species from the given species is $\stackrel{+}{ C } H _3$.
This because it does not contain $Cl$ atom, which exhibits electron withdrawing inductive effect. Carbocations are destabilized by -1 inductive (electron withdrawing) effect. When more number of-I groups are attached to the positively charged carbon atom, the positive charge on the carbon atom increases further, thus destabilizing the species. Hence, the species with no -I groups will be most stable.