Decreasing order of basicity is as follows CH3−>NH2−>CN−>HS− To determine the basic strength of these anions we need to find out the acidic strength of the conjugate acids. Conjugate acids of CH3−,NH2−,CN−,HS−are respectively CH4,NH3,HCN,H2S. As we know that, Acidity ∝ Electronegativity On going from left to right in the period, electronegativity increases. So, acidity order is HS−>CN−>NH2−>CH3−. Hence, the basicity order of the given anions will be just reverse of the acidity order of the conjugate acids.