Final answer:
The correct answer is a) Incentive, which is something that encourages people to take action, often used to increase participation by offering rewards like payments or prizes.
Step-by-step explanation:
One way to increase the number of responses is by providing an incentive such as payment or entry in a drawing. The correct answer is a) Incentive. An incentive is something that motivates or encourages someone to do something. Offering a payment or prize as an incentive can make it more attractive for individuals to participate. In contrast, an obligation is a duty or commitment, and providing an obligation would not typically increase the number of responses in a positive manner. It can be monetary or non-monetary, and it gives individuals a reason to participate.