Final answer:
Jean Harlow and Joan Crawford, movie stars of the 1930s, made nail polish popular by promoting it as part of their glamorous image.
Step-by-step explanation:
The two movie stars who are often credited with making nail polish popular in the 1930s were Jean Harlow and Joan Crawford. These screen sirens, known for their glamorous on-screen personas, captivated audiences and popularized many fashion trends, including colorful nail polish. Their influence on beauty and fashion trends made painted nails a must-have accessory for women aspiring to emulate their favorite stars.