Complementation tests are used to determine whether:

a. a new mutant phenotype of interest is dominant or recessive to another mutant phenotype.
b. two phenotypically identical mutants are mutations in the same gene or in different genes
c. two or more mutants have the same sequence variation (the same DNA sequence)
d. b and c
e. a, b, and c
f. a and c

i believe it's c. the trait for large beaks will be passed on to future generations through natural selection.

since eventually smaller beaks would have to find another source of food or even eventually just die out from the lack of food.

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).