Final answer:
In-depth interviews are the most commonly used selection technique and provide insights beyond what other methods like written tests or psychological evaluations can offer.
Step-by-step explanation:
Among all selection techniques, the single most commonly used, whether by itself or in conjunction with other tools, is C. in-depth interviews. In-depth interviews serve as a dynamic source of information about the candidate, providing insights into social factors, body language, and other nonverbal behaviors, which are not as readily captured through written tests, references, psychological testing, or resumes. Though other testing methods such as job-specific skills tests, personality tests, IQ tests, and even physical tests are utilized depending on the position, in-depth interviews remain central to the selection process for most mid-size to large-size businesses. They allow the employer to align the candidates' knowledge, skills, and abilities (KSAs) with the job's requirements, backed by research in industrial psychology and findings from job analysis.