In genetic engineering, a DNA segment (gene) of interest, is translated to the host cell through a vector. Consider the following four agents (A-D) in this regard and select the correct option about which one or more of these can be used as a vector/vectors
Statement
(A) A bacterium (B) Plasmid
(C) Plasmodium (D) Bacteriophage

Options

A. (i), (ii) & (iv)
B. (i) only
C. (i) & (iii)
D. (ii) & (iv)

Answer

To determine which of the given agents can be used as vectors in genetic engineering, we need to analyze each option:

1. **Agent A: Bacterium**
- Bacteria themselves are not typically used as vectors. They can serve as host cells for the expression of recombinant DNA, but they do not function as vectors to carry DNA segments into other cells.

2. **Agent B: Plasmid**
- Plasmids are small, circular DNA molecules that can replicate independently of chromosomal DNA within a bacterial cell. They are commonly used as vectors in genetic engineering because they can carry foreign DNA into host cells, allowing for the expression of the gene of interest.

3. **Agent C: Plasmodium**
- Plasmodium is a genus of parasitic protozoa, which causes malaria. It is not used as a vector in genetic engineering. Instead, it is a pathogen that affects host organisms.

4. **Agent D: Bacteriophage**
- Bacteriophages are viruses that infect bacteria. They can be used as vectors to introduce foreign DNA into bacterial cells. This makes them suitable for genetic engineering applications.

Based on this analysis, the correct options that can be used as vectors are **B (Plasmid)** and **D (Bacteriophage)**.

### Final Answer:
The correct options are **B (Plasmid)** and **D (Bacteriophage)**.