Final answer:
The molar with a distal cusp and unique developmental grooves is the first molar (A), typically appearing between the ages of 12 to 16 months. This tooth plays a key role in the dental occlusion and alignment of dental arches.
Step-by-step explanation:
The molar that is characterized by a distal cusp, often referred to as a fifth cusp or Cusp of Carabelli, and unique developmental grooves is the first molar (A). The first molars, also known as six-year molars, generally erupt between the ages of 12 to 16 months and are among the most important when it comes to dental occlusion due to their broad chewing surfaces and significant role in the alignment of the dental arches.
Third molars, commonly known as wisdom teeth, emerge much later in life and occasionally they can also have an extra cusp, but the question specifically refers to developmental grooves not found in other molars, which strongly indicates the first molars. Fourth molars do not exist; humans ordinarily have only three molars in each quadrant of their mouth.