Final answer:
The word 'Troubled' best describes the relationship between the voice and Mr. Thomas, given the emotional distress and effort to reassure, suggested by the narrative.
Step-by-step explanation:
The relationship between the voice and Mr. Thomas would best be described as troubled. This is evidenced by the description of the Doctor looking 'vexed, puzzled', which suggests a state of confusion or concern possibly caused by a severe problem indicated by 'Some terrible problem lay hid in this woman's face.' Furthermore, the reference to Bess trying 'to whisper with trem-ulous lips' while looking at someone with 'sad, dark eyes' implies an emotional distress, and the effort 'to reassure him' indicates a troubled scenario, where Mr. Thomas is seeking comfort or answers. Therefore, out of the provided options, 'Troubled' (D) most accurately describes the relationship in question.