The equable climate and open grass vegetation free of tsetse flies, are responsible for the large scale movement of nomadic cattle during the dry season to the unit marked

Here are the solutions to the questions based on the provided map of Nigeria's vegetation belts:

16. The equable climate and open grass vegetation free of tsetse flies, are responsible for the large scale movement of nomadic cattle during the dry season to the unit marked
Nomadic cattle, primarily kept by the Fulani, thrive in open grasslands and avoid areas infested with tsetse flies. The Sudan Savannah (region 6 on the map) is characterized by open grass vegetation and is largely free of tsetse flies, making it a primary area for cattle rearing and a destination for nomadic pastoralists during the dry season when they move from even drier northern regions.
The correct option is B.
The unit marked is $\boxed{\text{B. 6}}$.

17. The Vegetation belt numbered 4 on the map is the
Region 4 is located between the tropical rainforest (further south) and the Guinea Savannah (further north). This transitional zone, often a result of human activities like farming and burning degrading the original forest, is known as the Derived Savannah or Forest/Savannah Mosaic.
The correct option is B.
The Vegetation belt numbered 4 on the map is the $\boxed{\text{B. Derived savannah}}$.

18. The largest concentration of domesticated livestock in Nigeria is found in the vegetation zone marked
The northern savannah zones of Nigeria, particularly the Sudan Savannah (region 6), are known for having the largest concentration of domesticated livestock, especially cattle. This is due to the extensive open grasslands suitable for grazing and the relative absence of tsetse flies, which transmit diseases to livestock.
The correct option is B.
The largest concentration of domesticated livestock in Nigeria is found in the vegetation zone marked $\boxed{\text{B. 6}}$.