Answer :
I think that Indra Nooyi is using social power because she involves the consumers in creating innovation. She makes a close interaction based on the feedback of the consumers. And as one community, consumers have also the right to propose solutions for continuous development. All the other choices do not conform to her Pepsi challenge. She does not use her personal power that may include her intimidating people because of her reputation. It could also not be legitimate power because there is no legal actions that are used. It also can't be expert power because she just asks for open ideas, and not offer to them her expertise of her field.