Final answer:
The symbol uₚ refers to the population proportion, and the correct answer to what is the uₚ is c. 0.72, which is the sample proportion of students spending over 3 hours a day on their phones in decimal form.
Step-by-step explanation:
The question asks about the proportion of students who surveyed spent over 3 hours a day on their phones. According to the Gallup poll, 72% of the 300 students, which represents the sample proportion, spent this amount of time. Therefore, the symbol uₚ stands for the population proportion, and since we're given a sample proportion instead of the population proportion, the correct answer among the provided options is c. 0.72, which represents the percentage of students in decimal form who spend over 3 hours a day on their phones.