Final answer:
The diary method is commonly used alongside C) observational research and surveys in social studies to provide a comprehensive view of individual behaviors and opinions.
Step-by-step explanation:
The diary method is a qualitative research tool often used in social studies to gather in-depth information about individuals' day-to-day experiences and activities.
It can provide rich, detailed data about a person's behaviors, thoughts, and feelings over time. When we consider the options provided and compare them to the methods used in conjunction with the diary method, the most fitting answer is that the diary method is also used in conjunction with C) observational research; and surveys.
This is because observational research involves systematically watching and recording behaviors and events as they occur naturally, while surveys collect self-reported data that can complement the personal insights derived from diary entries.