Answer
(a) positive
(b) zero
Work Step by Step
(a) We know that the pendulum moves from point I to point II in a downward direction -- that is, in the direction of gravity. Hence, the work done on the bob by the gravity is positive.
(b) As the bob swings from point I to point II, the tension in the string acts in a perpendicular direction to the motion of the bob. Thus, the work done on the bob by the string is zero because $W=Fdcos90^{\circ}=Fd(0)=0$
