Question

what could increase the fitness of a polar bear according to darwins theory? being unable to reproduce having a thicker coat of fur having black fur in the artic

Explanation:

According to Darwin's theory, fitness refers to an organism's ability to survive and reproduce in its environment. A thicker coat of fur would help a polar bear better survive in its cold Arctic habitat (providing better insulation), which in turn increases its chances of living long enough to reproduce and pass on its genes. Being unable to reproduce directly reduces fitness as it can't contribute to the next generation. Having black fur in the Arctic would likely be a disadvantage (as white fur provides camouflage in the snow - ice environment), reducing survival and thus fitness.

Answer:

having a thicker coat of fur