A lateral flow test strip (such as a home pregnancy test used to detect the hormone hCG\text{hCG}hCG) is a crucial diagnostic tool for monitoring health. The diagram below illustrates the spatial arrangement of antibodies on such a test strip:
Which of the following correctly describes the role of the immobilised monoclonal antibodies located specifically at the Test Line?
They are immobilised monoclonal antibodies specific to a different epitope on the target analyte, trapping the mobile-antibody-analyte complex to produce a coloured line.
They are mobile monoclonal antibodies conjugated to pigment particles that bind directly to the target analyte upon sample application.
They are immobilised antibodies that bind specifically to the constant region of mobile antibodies, serving as a control to verify test validity.
They are catalytic monoclonal antibodies that enzymatically degrade the target analyte to trigger a localised physical colour change.