Answer:
C) When researching the health effects of drinking soda, you decide to research the effects of drinking only diet soda.
Explanation:
Narrowing a topic is a step towards a deep and detailed analysis of a certain topic. In this process, a topic is focused and narrowed so that it can be deeply explored in details. This helps in the researcher to move in a specific way of research and not deviate in the process. Developing research questions, avoid generalized topics and forming subtopics are some of the ways in which the narrowing of the topics are done.
In the given sentence, the topic is narrowed by asking the targeted question and focusing only on the effects of diet soda.