The function of Phospholamban is: (A) Regulates Na+ K+ pump, (B) Transports calcium out of the mitrochondria, (C) Binds actin with myosin, (D) Inhibits Ca++ pump on sarcoplasmic reticulum
Answer is D. Phospholamban inhibits calcium pump on sarcoplasmic reticulum, thereby preventing uptake of Ca++ from the sarcoplasm. 
When it is phosphorylated, the Ca++ channels open up and uptake of Ca++ is increased into the sarcoplasmic reticulum.