In case of COVID positive patients, presence of corona virus is suspe…

The correct answer is Option (3) → Reverse transcriptase Polymerase Chain Reaction (RT-PCR)

Before symptoms appear, a pathogen is present only in very small amounts. PCR can amplify its nucleic acid until it can be detected. The corona virus has an RNA genome, so its RNA is first copied into DNA by reverse transcriptase, and that DNA is amplified by PCR.

Option (1) Enzyme linked immuno-sorbent Assay (ELISA) only — Incorrect. ELISA detects antigens or antibodies; the antibodies take time to form, so it is less useful before symptoms.

Option (2) Recombinant DNA technology only — Incorrect. This is a broad technology, not a particular test for the virus.

Option (3) Reverse transcriptase Polymerase Chain Reaction (RT-PCR) — Correct. It finds even a tiny amount of viral RNA, so it works before the disease shows.

Option (4) Widal Test — Incorrect. The Widal test confirms typhoid fever. Typhoid is caused by the bacterium Salmonella typhi, not by a virus. So this test cannot detect the corona virus.

NCERT Extract:
Molecular Diagnosis – “… Using conventional methods of diagnosis (serum and urine analysis, etc.) early detection is not possible. Recombinant DNA technology, Polymerase Chain Reaction (PCR) and Enzyme Linked Immuno-sorbent Assay (ELISA) are some of the techniques that serve the purpose of early diagnosis. Presence of a pathogen (bacteria, viruses, etc.) is normally suspected only when the pathogen has produced a disease symptom. By this time the concentration of pathogen is already very high in the body. However, very low concentration of a bacteria or virus (at a time when the symptoms of the disease are not yet visible) can be detected by amplification of their nucleic acid by PCR. … PCR is now routinely used to detect HIV in suspected AIDS patients.”
Common Diseases in Humans – “… Typhoid fever could be confirmed by Widal test.”