Final answer:
In fabricating an oral appliance, it is necessary to replicate the gingival contours of the teeth and the soft palate for a comfortable and functional fit. Other structures like the sulcus and periodontal ligament are not typically replicated in the appliance.
Step-by-step explanation:
When fabricating an oral appliance, the following oral anatomical structures should be necessarily replicated to ensure a precise and comfortable fit: (c) Gingival contours of the teeth, which includes ensuring that the gums surrounding the teeth are accurately represented to prevent irritation and ensure proper fit; and (d) Soft palate, as it is crucial for the appliance to accommodate the movement and function of this area without interfering with actions like swallowing or speaking.
The other options like (a) Sulcus and (b) Periodontal ligament are not typically replicated in oral appliances. The sulcus is the space between the tooth and the gum but isn't directly molded in the appliance, and the periodontal ligament's function and precise replication aren't required within an oral appliance's structure, given that it serves to connect the teeth to bone and isn't part of the appliance's contact with the mouth.The gingival contours of the teeth refer to the shape or contour of the gums around the teeth, which should be replicated for aesthetic and functional reasons.