Work Rules are hard constraints that filter candidates in or out, while Service Objectives are soft constraints that rank the remaining candidates. The 40-mile radius is a strict eligibility rule, while preferring contractors over internal employees is a preference.

Option D is correct. A Work Rule (such as Maximum Travel From Home with a Relevance Group for contractors) enforces the 40-mile radius as a hard filter --- contractors beyond that distance are excluded from the candidate list. A Service Objective (such as a Resource Preference or Resource Priority objective) is the correct mechanism to rank contractors above internal employees while still allowing internal employees to be considered when no contractor is available.

Option A is incorrect because a Service Objective cannot enforce the maximum travel as a hard rule; it would only score it.

Option B is incorrect because using a Work Rule for the contractor preference would exclude internal employees, violating requirement #1.

Option C is incorrect because the roles of the rule types are reversed --- travel limit needs to be a Work Rule and preference needs to be a Service Objective.