What could cause 'S' sounds to sound like 'th'?

The phenomenon where 'S' sounds are pronounced more like 'th' can primarily arise from dental and palatal positioning. When the maxillary anterior teeth are positioned too far palatal, it can obstruct the proper airflow and tongue positioning necessary to produce the clear 'S' sound. The interference from the dental arch can alter the tongue's placement, which is crucial for the correct articulation of sibilant sounds.

In addition, if the palate itself is too thick, this can also lead to similar issues. A thicker palate may impede tongue movement or change the acoustics of the sound being produced, causing 'S' sounds to lose their sharpness and clarity, morphing them instead towards the 'th' sound.

Choosing the answer that includes both the positioning of the maxillary anterior teeth and the thickness of the palate recognizes that either factor could individually or collectively contribute to the issue. Therefore, option D correctly encapsulates the causes that could lead to this articulation challenge by acknowledging the contributions of both anatomical variations.