Answer
The correct answer is option B. A COVID-19 Rapid Antigen Test is an example of a lateral flow assay. When a positive sample is added to the test well and starts to flow along the device, the labelled primary antibody binds to the antigen. The immobilised unlabelled primary antibody captures the antigen bound to the labelled primary antibody, generating the first visible (test) line in the viewing area. The immobilised unlabelled secondary antibody captures the labelled primary antibody, generating the second visible (control) line in the viewing area.