In aqueous solution the configuration of the ions are following Cr3+(21) V3+(20)Tr3+(19)Mn3+(22)=3d34s0⇒t2g3eg0=3d24s0⇒t282eg0=3d14s0⇒t281eg0=3d44s0⇒t2g4eg0 We know that half-filled and fully filled orbitals are more stable than partially filled orbitals. Hence among the given ions the only Cr3+ ion which occupies half-filled t2g orbitals. Caution Although Mn3+ ion has half-filled ts orbital but there is one more electron filled in eR orbital which reduces the stability.