Final answer:
The voluntary, professional, and egalitarian relationship is best described as a Therapist-Client Relationship, where both parties contribute to the therapeutic process on an equal basis.
Step-by-step explanation:
The description of the relationship as voluntary, professional, and egalitarian suggests a partnership where both parties interact on an equal and mutually beneficial basis. Considering the options provided, d) Therapist-Client Relationship best fits this description, as it typically embodies a structure where therapist and client work together toward the client's well-being, with respect and an understanding that both contribute to the therapeutic process. Doctor-Patient and Supervisor-Subordinate relationships generally have an inherent power differential, and while a Teacher-Student relationship is professional, it is not typically described as egalitarian in the same sense as a therapeutic one.