Which of these choices is the main threat to the survival of polar bears?

d. warming of the frozen Arctic,

The loss of sea ice habitat caused by climate change is the most important threat to the survival of polar bears.  Other big threats include unsustainable hunting,  polar bear-human conflicts, and industrial impacts.

Loss of sea ice habitat will result from d.warming of the frozen Arctic which will cause the winter ice pack to shrink and break up earlier thus limiting survival of polar bears. 

Melting of ice in polar bear's hunting territory. (APEX)