I believe the most appropriate answer would be to change the core from wood to iron. This is because iron is a magnetic material while wood is not magnetic hence cant acquire magnetism. Other factors that would increase the strength of electromagnet would be; increasing the amount of electric current, and increasing the number of windings.
The answer is: Survival of the form that will leave the most copies of itself in successive generations.
"Survival of the fittest" is a phrase that originated from Darwinian evolutionary theory.
This is example of natural selection and adaptation.
Genetic variation is important to the population's ability to survive in different situations that affect natural selection.
The environment is constantly changing and different alleles are favored.
What was the question here?