$$\textbf{Correct answer: B}$$
$$\textbf{Option (A):}$$
$$\bullet$$ Genetically modified animals may have specific genetic alterations or gene insertions in a few cells. But this is not the condition for a transgenic animal.
$$\bullet$$ This type of gene alteration in a specific region is done by specifically targeting the genome of tissues in a particular organ or system.
$$\textbf{Option (B):}$$
$$\bullet$$ Transgenic animals are those with foreign DNA inserted in all their cells.
$$\bullet$$ This is because the foreign gene is incorporated into the genome of the organism from its zygotic stage.
$$\bullet$$ By using the microinjection procedure the foreign gene can be inserted into the zygote.
$$\bullet$$ Since all the cells of the organism are produced from the zygotic cell, all the cells in the body of the transgenic animal will have the gene insert.
$$\textbf{Option (C):}$$
$$\bullet$$ A foreign RNA is not introduced in a cell as it is more unstable and even if it remains viable it will only produce proteins for a shorter period.
Hence, the correct answer is option (B).