1 It is effecient,because it doesn't waste any resources.
2 the monoploly determines price and quantity according to the principle of MC=MR.
3 generally speaking,MPl/MPk=w/r,w is wage and r is the interest.However,in this case,q=L=4K,L/K=4/1,so the L is 25.6 and K is 6.4