Answer:
D. Stratified sampling
Step-by-step explanation:
Stratified sampling is a sampling method where the population is divided into subgroups and data is sampled randomly at each subgroup proportional to the size of the subgroups.
Then all sampled data is combined to create the population sample for the study.
Debra collected 9 power usage data, 3 random power usage data from each 3 subgroups: power morning, aftemoon, and evening power usage. She performs stratified sampling method.