Well can a bacterium be used as a vector?I have learnt that Agrobacterium tumifacian can be used to deliver a gene of interest in plant cells but on a book in a multiple choice question it says that only plasmid and bacteriophage can be used and not the bacteria to deliver a gene of interest to a host cell.
Yes, Agrobacterium is indeed a very widely used vector in plants. So it wouldn't be wrong to consider bacteria as vectors. Just to add, it's worth noting that in recent times, bacterial vector options have been explored in the case of humans also, especially in the case of gene therapy for cancer treatment, though its success hasn't been demonstrated yet. Refer https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3056088/ for more information on this.
To conclude, bacteria can be considered as a vector undoubtedly.
Agrobacterium tumefaciens and Agrobacterium rhizogenes are soil-based plant pathogenic bacterial strains containing plasmid. This plasmid is known as Ti plasmid and is responsible for inducing tumor. Part of this plasmid called T-DNA can be integrated into the host chromosomes. So, this bacterium plasmid act as vector, but not the whole bacterial cell.
This is true with all the pathogenic bacterial strains, as they can transfer their DNA material to cause pathogenicity. So, these pathogenic strains can be used in genetic engineering to transfer particular gene to host/mammalian cell. Using bacteria as a vector to treat cancer or various other pathological conditions is still at developmental stage.
The conceptual mistake that you're making is that, Agrobacterium Tumifaciens, being a bacterium, contains additional genetic material within its cell, called the 'Ti plasmid'. Hence it is quite justified that the plasmid within the bacterium is acting as a vector, or a carrier of a foreign gene and not the whole bacterium itself.
Hence the answer provided in the book is correct.