Final answer:
Option (B), An omnibus survey is a cost-effective research method where multiple clients add questions to a shared survey on a variety of topics, often used in market research.
Step-by-step explanation:
When a research agency conducts a routine survey allowing respondents to participate voluntarily and adds a small number of questions to the existing survey for cost-effective data, this is an example of a(n) omnibus survey. An omnibus survey is a type of survey that includes a range of topics and is shared among multiple clients who each add their own questions, thus reducing the overall cost of research.
Unlike surveys that are exclusively designed for sociological research or specific scientific inquiry, omnibus surveys focus on a variety of issues and are often used for market research and gathering consumer attitudes.