Which of the following best describes the concept of replication in experimental design?

A city is divided into $120$ neighborhoods after a major flood. Twenty neighborhoods are chosen at random, and every resident in those neighborhoods is surveyed about their immediate needs. What sampling method is used, and what is a possible source of bias in this approach?

In a clinical trial, $80$ patients are numbered from $1$ to $80$. The trial requires $40$ patients to be randomly selected for the intervention group. Which method is most appropriate for selecting the intervention group?

A team of researchers is investigating the impact of a new concentration-enhancing supplement on college students. A total of $40$ students aged $18$–$20$ agree to participate. The students are randomly assigned to receive either the supplement or a placebo daily for two weeks. Their performance is assessed using standardized focus and attention tests at both the beginning and the end of the study. How could blocking be used in the design of this experiment to improve its accuracy?

A medical research firm obtained a nationwide registry of $N=4800$ adults diagnosed with Type II diabetes and needs a sample of $n=40$. The registry contains counts by age strata: $18\text{–}39$ has $N_1=1200$, $40\text{–}59$ has $N_2=2000$, and $60+$ has $N_3=1600$. Which sampling design and selection procedure is most appropriate to obtain a representative sample of $n=40$?