The question is asking which group's merchants dominated trade, including the sale of slaves, within their land. The Igbo people were significantly involved in internal and external slave trade, sup

The question is asking which group's merchants dominated trade, including the sale of slaves, within their land. The Igbo people were significantly involved in internal and external slave trade, supplying many slaves to the coast.
The correct option is (c).

$$\boxed{\text{(c) Igbo}}$$

European merchants used their ships as trading stations and also established warehouses on land or on hulks for storing goods in the delta.
The correct option is (a).

$$\boxed{\text{(a) warehouses}}$$

The actual buying and selling of human beings, where one has absolute possession of another, is defined as Slave Trade.
The correct option is (b).

$$\boxed{\text{(b) Slave}}$$

The legal trade established after the abolition of the slave trade is known as Legitimate Trade.
The correct option is (b).

$$\boxed{\text{(b) Legitimate}}$$

The campaign against the slave trade, which led to its abolition in 1807 and slavery in 1833, was primarily led by Britain.
The correct option is (c).

$$\boxed{\text{(c) Britain}}$$

Up to 1850, palm oil was continuously exported from West Africa, primarily by the people of the Delta region (Niger Delta).
The correct option is (c).

$$\boxed{\text{(c) Delta}}$$

Up to 1850, slaves were exported from the Niger Delta region in West Africa.
The correct option is (d).

$$\boxed{\text{(d) Niger Delta}}$$

The transatlantic slave trade, which started in West Africa, covered a period of approximately 400 years (from the 16th to the 19th century).
The correct option is (b).

$$\boxed{\text{(b) 400years}}$$