Final answer:
The distolingual cusp of the maxillary first molar opposes the mesial fossa of the mandibular first molar in an ideal intercuspal position.
Step-by-step explanation:
The question relates to occlusion in dentistry, specifically the alignment of teeth when the jaw is closed an ideal intercuspal position (ICP). The distolingual cusp of the maxillary first molar opposes the mesial fossa of the mandibular first molar in an ideal intercuspal position.
This occurs when the upper and lower teeth fit together in the most optimal way during occlusion. The 'DL cusp of max 1st molar' refers to the distolingual cusp of the maxillary first molar. In an ideal intercuspal position, this cusp would oppose the mesial fossa of the mandibular first molar, which is the depression on the mesial side of the lower first molar.