Final answer:
The correct method to introduce a plasmid into E. coli is by transformation. It involves taking up free DNA from the surrounding environment by bacteria, as opposed to microinjection which is used for eukaryotic cells. the correct option is A. Transformation.
Step-by-step explanation:
The method used to introduce the plasmid into E. coli is transformation. Transformation is a commonly used laboratory technique in which bacteria take up free DNA from their surroundings, which in a lab setting is often in the form of recombinant plasmids. This process allows for the bacterial uptake of foreign DNA leading to a genetic change.
The technique of microinjection, on the other hand, is used to introduce DNA fragments directly into the cytoplasm or nucleus of eukaryotic cells using a glass micropipette. It is not the method employed for E. coli, which is a prokaryote. Similarly, gene guns and Ti plasmids with Agrobacterium are generally used for transfecting eukaryotic hosts such as plants.