Final answer:
Dressing or shaping lotion is used to keep the base flat while creating an indentation pin curl, not hair spray, which is more suitable for holding hairstyles in place. option d.
Step-by-step explanation:
The product used to keep the base flat while curving the stem with a comb when forming an indentation pin curl is D) Dressing or shaping lotion. While hair spray is commonly used to hold hairstyles in place and can stiffen hair, it is not the most effective choice for keeping the base flat during the pin curl process. Dressing or shaping lotion is specifically designed to help mold and sculpt the hair, making it an ideal choice for maintaining the shape of the curl and keeping the base flat.