Soil compaction results from
b. the weight of heavy machinery on soil
c. livestock walking over soil
d. all of the above

Choices on the given question are as follows:
a.the reduction or elimination of pore space in soil
b.the weight of heavy machinery on soil
c.livestock walking over soil
d.all of the above

The answer is letter is letter d. all of the above. 
Soil compaction happens when the soil particles are pressed together, reducing pore space between them. Wheels of heavy machinery like tractors is one of the major causes of soil compaction.

Carbon monoxide is the answer: )