How might researchers identify which ion enters the cell from outside, causing the rapid rise in membrane potential that initiates the action potential in additional experiments

not: both offer magnification to the atomic level.

the last one: artificial selection/genetic engineering

plant breeding is modifying the genetic composition of the plant - but this isn't cloning (which is specifically introducing a gene into a genome). but you are artificially selecting for the phenotype that you want.

the second one is literally genetic engineering - the gene would have been cloned first and then inserted or genetically engineered into the new plant cell (the cotton).