Final answer:
Option B, 'an observational study', corresponds to well-designed controlled trials without randomization. Observational studies measure outcomes without manipulating variables, appropriate when randomization is not possible or ethical. These include cross-sectional, case-control, and cohort studies.
Step-by-step explanation:
Evidence from well-designed controlled trials without randomization falls under the category of quasi-experimental research. Quasi-experimental studies share similarities with experimental research but lack random assignment of participants to groups. While experimental research (option b) involves randomization, quasi-experimental designs aim to investigate causal relationships in real-world settings where random assignment may not be feasible or ethical. These studies employ control groups and intervention groups, allowing researchers to assess the impact of interventions or treatments. Cross-sectional studies (option c) and descriptive research (option d) do not specifically address the experimental or quasi-experimental manipulation of variables and are more focused on observing and describing phenomena. Meta-analysis (option a) involves synthesizing results from multiple studies but may include various research designs, including quasi-experimental ones.assess the natural differences in nutrition and school performance, without actively manipulating the children's diets.