Final answer:
Cepheid stars are yellow supergiant pulsating stars, not any of the other options listed in the question. They are crucial for astronomical measurements due to their predictable luminosity-period relationship.
Step-by-step explanation:
Cepheid stars are not white dwarf stars, young stars, or members of binary systems necessarily. Instead, option d. stars that pulsate in brightness, size, and temperature is the correct choice. Cepheids belong to a class of yellow supergiant pulsating stars which exhibit a consistent relationship between their pulsation periods and luminosities—a property that makes them valuable for measuring astronomical distances. These stars do not permanently stay in this phase but are going through an unstable stage in their stellar evolution.