Step 1: Define bioassay.
A bioassay (biological assay) is a method of measuring the potency or concentration of a substance by observing its effect on a living system. This could be a whole animal, isolated tissue, cell culture, or microorganism.
Step 2: What makes bioassay unique?
Unlike chemical or spectroscopic assays, bioassay does not rely on chemical structure, molecular weight, or spectral absorption. Instead, it uses the biological response (e.g., contraction of a muscle, inhibition of bacterial growth, change in blood sugar) to quantify the substance.
Step 3: Why is it used?
Bioassays are used when a substance is too complex to be assayed chemically, or when its activity must be confirmed in a biological system. For example, insulin, heparin, and oxytocin are standardised by bioassay.
Step 4: Eliminate wrong options.
Chemical structure and molecular weight are physical/chemical properties, not the basis of bioassay. Spectral absorption belongs to UV/Vis/IR spectroscopy.
Step 5: Conclusion.
Bioassay is fundamentally based on the biological response of a living system to the test substance.
Answer: Option (2) — Biological response