Answer:
The method of data collection in this situation is an a) experiment study.
Step-by-step explanation:
Consider the provided information.
Observational study: In this study observer only observe the subjects, and measure variables of interest without allocating treatments to subjects.
Experiment study: The researchers are applying treatments to experimental units in the research, then the effect of the treatments on the experimental units is observed.
Here the manufacturer apply treatment and observe the effect of "Rust Away" (chemical agent) on metallic goods.
The study is controlled so that manufacturer understand cause-and-effect of new chemical agent.
Hence, the method of data collection in this situation is an a) experiment study.