The correct option is C (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii)
(a) Bt cotton is a transgenic crop that produces the same toxin as the bacterium Bacillus thuringiensis in the plant cells, thereby protecting the crops from pests. The bacterium secretes specific proteins known as “Cry proteins” that are toxic to insects.
(b) Gene therapy is a technique to replace a faulty gene by a normal gene which will help to restore normal functioning of the gene. In order to treat adenosine deaminase deficiency, gene therapy is used widely.
(c) RNAi (RNA interference) is a biological process in which RNA molecules inhibit gene expression or translation, by neutralizing targeted mRNA molecules. It takes place in all eukaryotic organisms as a method of cellular defense.
(d) AIDS is caused by HIV, which is a retrovirus (viruses containing RNA as genetic material). Hence, RT-PCR is used for its detection as it amplifies even a single copy of RNA to produce millions of copies of cDNA. Thus, the presence of even a tiny number of viruses can give a positive result, thereby allowing its early detection.