(a) The silk cloth and woolen cloth becomes negatively charged because when an object is charged by rubbing against another object the two objects gets oppositely charged. And by convention, it is considered that charge acquired by cloth is negative.
The figure below illustrates more:
(b) The glass rod and balloon becomes positively charged due to this when two balloons rubbed with the woolen cloth are brought close to each other repel each other as shown in the figure below.
