Concept:Animal traction depends on healthy draught animals, and disease-spreading insects can make this impossible.
Explanation:The forest zones of West Africa provide a favourable environment for the tsetse fly.
The tsetse fly transmits trypanosomiasis, commonly known as sleeping sickness, to cattle.
Infected cattle become weak and may die, so farmers cannot keep them for ploughing or farm work.
Therefore, the presence of tsetse flies is the major factor that limits animal traction in these forest zones.
Answer:B. presence of tsetse flies