Which one of the following vectors is used to replace the defective gene therapy?

The correct option is C Adenovirus
Gene therapy
Gene therapy is the collection of methods in which a defective gene present in the embryo or child is corrected by replacing it with a normal functional gene to compensate for the malfunctioning of the defective gene.

Adenovirus
Adenovirus is one of the viruses which has DNA as genetic material. It is modified in such a way that the essential gene required for replication is replaced with foreign gene of interest.
Thus, they are made noncapable to replicate inside a host cell and can be used as a vector to transfer the correct gene in gene therapy.

Cosmid
A cosmid is a plasmid that forms a hybrid with the lambda phage cos sequence and is commonly used as a cloning vector in genetic engineering.

Ri plasmid
Ri plasmid is a hairy root inducing plasmid obtained from Agrobacterium rhizogenes that induces plants to produce hairy roots.

Ti plasmid
Agrobacterium tumefaciens is a bacterial plant pathogen which carries the Ti plasmid or tumour inducing plasmid (extrachromosomal DNA). The T-DNA segment of the Ti plasmid integrates itself into the plant cell’s genome to induce tumour formation in plants.

This property of the plasmid makes it an effective vector for transferring foreign genes into plant cells. The vector is modified such that it is no more pathogenic to the plants but is still able to use the mechanisms to deliver genes of our interest into a variety of plants.

Final answer:
(a) Adenovirus