Final answer:
The correct answer is option 1: the parts of a pin curl are the base, stem, and circle, each contributing to the direction, mobility, and form of the curl.
Step-by-step explanation:
The three principal parts of a pin curl are base, stem, and circle. Option 1 is correct. The base is the area of the scalp where the curl is situated, it determines the direction and movement of the curl. The stem, also known as the arc or loop, is the section of hair between the base and the first turn of the curl, which gives the curl its direction and mobility. Lastly, the circle is the part of the curl that forms the complete turn(s) and actually creates the wave or curl in the hair.