An ELISA can be used for:

• A. Quantitative analysis.
• B. Size analysis.
• C. Sequence analysis.
• D. Structure analysis.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand what ELISA (Enzyme-Linked Immunosorbent Assay) is used for in microbiology. ELISA is a biochemical technique primarily used to detect the presence and quantify the amount of antigens or antibodies in a sample.
2. Step 2: Recognize that ELISA provides quantitative data by measuring the concentration of a specific protein or molecule through an enzyme-substrate reaction that produces a measurable color change.
3. Step 3: Differentiate ELISA from other types of analyses: it does not provide size analysis (which is done by techniques like gel electrophoresis), sequence analysis (done by DNA sequencing methods), or structure analysis (done by methods like X-ray crystallography or NMR).
4. Step 4: Conclude that the primary application of ELISA is quantitative analysis, as it allows researchers to determine how much of a particular antigen or antibody is present in a sample.
5. Step 5: Summarize that ELISA is a powerful tool for quantitative measurement in immunology and microbiology, making 'Quantitative analysis' the correct choice among the options.