Correct Option: B

In heart cells cAMP acts as secondary messenger which is made in the response of adrenaline and it stimulates Ca²⁺ ions to come out from the sarcoplasmic reticulum of muscle fibres which causes muscle contraction. cGMP (Cyclic Guanosine Monophosphate) which acts as secondary messenger works on the action of acetylcholine, increase in flow of Ca²⁺ into muscle fibres & hence causes muscle relaxation. There is no role of sodium in hormonal action.