The correct answer is B. Number of customer contacts . In a call center, overhead is driven primarily by the volume of customer interactions handled, so the most appropriate cost driver is the number of customer contacts or calls. Cost-per-call and contact-center cost analysis commonly use the number of calls or contacts as the central activity measure because those interactions consume staff time, telecom systems, and support resources.

Option A, total material cost , is not appropriate because call centers are service operations and usually do not consume direct materials in the way manufacturers do. Option C, total sales dollars , may be relevant for some selling analyses but does not directly measure the activity causing most call center overhead. Option D, number of labor hours , can sometimes be useful, but in this setting the more direct activity driver is the actual number of contacts handled. Since overhead in a call center tends to rise with customer interactions, the best allocation base is the number of customer contacts . Therefore, Option B is the correct answer.