when a conducting soap bubble is negatively charged then

a. its size starts varying arbitarily

b. it expands

c. it contracts

d. no change in its size takes place

The soap bubble is conducting. So, when a negative charge is applied on it, this charge will distribute itself on the surface of the bubble.

So, there is a net negative charge on the soap bubble. Now, each individual charges will repel each other and the radius of the bubble increases.

Therefore, when a conducting soap bubble is negatively charged, it expands.