Final answer:
Star B, which has a longer period of 20 days, is more luminous than star A, which has a period of 10 days, due to the direct relationship between a Cepheid variable star's period and its luminosity. Therefore, the correct option is B.
Step-by-step explanation:
The question asks about the luminosity of two Cepheid variable stars based on their respective periods. According to the Period-Luminosity Relation, the luminosity of a Cepheid variable star is directly related to its period of luminosity changes. The longer the period, the greater the luminosity of the star. Therefore, for the two Cepheid variables in question, star A with a period of 10 days and star B with a period of 20 days, the correct statement is that star B is more luminous. This is because star B has the longer period, and according to the relation discovered by Henrietta Leavitt, a longer period indicates a higher average luminosity.