What does 'competent' refer to in competent cells used in transformation experiments?

The full form of $PCR$ is ...........

$I. ... A...$ is the ability of a cell to take up foreign $DNA$.
$II.$ The cell is treated with a specific concentration of a divalent cation such as $... B...$ to increase the pore size in the cell wall.
$III.$ In $...C...$ method,recombinant $DNA$ is directly injected into the nucleus of an animal cell.
The most appropriate option regarding $A, B$ and $C$ is:

The method in which recombinant $DNA$ is directly injected into the nucleus of an animal cell by using microneedles is called:

In a mixture,$DNA$ fragments are separated by

The technique of bombarding plant cells with high velocity microparticles of gold or tungsten,coated with $DNA$,is known as: