Final answer:
The high use of fossil fuels has likely contributed the most to the decline in population of polar bears.
Step-by-step explanation:
The human activity that likely contributed the most to the decline in population of polar bears is the high use of fossil fuels. Burning fossil fuels releases greenhouse gases, which trap heat in the atmosphere and contribute to global warming. As temperatures rise, sea ice melts, reducing the polar bears' hunting grounds and making it harder for them to find food.
Learn more about human activity and decline in population of polar bears