Gene transfer in plants can be achieved using several methods, including:
1. Agrobacterium-mediated transformation: A natural method where Agrobacterium tumefaciens is used to transfer genes into plant cells.
2. Electroporation: A method where plant cells are exposed to an electric field to facilitate gene uptake.
3. Microinjection: A method where DNA is directly injected into plant cells using a fine needle.
Thus, all of these are valid methods for gene transfer in plants. Therefore, the correct answer is:
\[ \boxed{(D) \, \text{All of the above}} \]