Answer:
Region A: 20,178 people/521 km²
= 38.7 people/km²
Region B: 1,200 people/451 km²
= 2.7 people/km²
Region C: 13,475 people/395 km²
= 34.1 people/km²
Region D: 6,980 people/426 km²
= 16.4 people/km²
The regions, in order from the most densely populated to the least densely populated: A, C, D, B