Final answer:
Focus groups are an expensive data collection method that allow for real-time synthesis of various viewpoints through direct interaction among participants, facilitated by a moderator.
Step-by-step explanation:
The data collection method which is expensive but allows for on-the-spot synthesis of different viewpoints through two-way communication is d) Focus groups. This method facilitates immediate interaction and discussion among participants, often yielding dynamic insight into the attitudes, perceptions, and opinions of a group concerning a particular topic. Focus group discussions are usually moderated by a researcher to probe deeper into the subject matter and capture a rich understanding of the participant's experiences and ideas.